The second ending for the anime, "Great Escape", has some masterfully done foreshadowing. At first, it shows Eren, Mikasa, Armin, Sasha, Connie, and Jean. People who would do anything to protect their friends and the people they care about. Then it shows Annie, Reiner, Bertolt, Ymir, and Krista. People who all have secrets to hide...
Of course, they want women in the military; the last thing the Walls need is a bigger population! And, you know, soldiers being scarce enough as they are, without any need of refusing to accept 50% of the population.
It's not likely tha the military is being used as a population control method...or, at least, not an effective one. I that were the case, then women wouldn't be allowed in the military, and soldiers would not be allowed to marry or have children. That would lessen the number of sexual partners either gender could have, and thus reduce the birth rate. If anything, accepting men and women into the military would encourage a more stable birth rate, allowing current workers to replace older workers as time goes by, keeping the economy stagnant but stable so that the rich can be safe and supported by the poor and nobody has to venture outside the walls more than absolutely necessary. Any excess mouths go to the military, regardless of gender; no one is any less expendable than anyone else.
Bertolt almost always has an awkward look about him. He's probably not used to infiltrating the ranks of humans as opposed to Reiner and Annie.
Reiner and Bertolt look younger than they should have, in Reiner's flashback to when he witnessed a Titan devour his friend. It's because it happened prior to them breaching the Wall and infiltrating the human ranks.
Why does the Colossal Titan appear on the exact day that Eren graduates from his training, five years after its first appearance? Because the Colossal Titan is Bertolt, another of the Trainees graduating and being inducted into the military. It's the first time he's had the freedom to access the Wall without arousing suspicion.
When Connie brings up what his group saw in his Doomed Hometown, Reiner tells him to be quiet, and Ymir jokes around to take his mind off of it, something Reiner asks her to keep doing so he won't worry. Since Reiner is the Team Dad, we can understand why. When The Reveal about him and Bertolt comes around, however, it becomes clear that his main concern was keeping Connie from thinking about the Titan on his house that resembled his mother and even greeted him like a mother would. Note how he only tells Connie to stop talking about it when that part of the story comes up.
Whose idea is it to save the rogue Titan, and who pipes up in favor? Reiner and Annie, respectively. Annie even appears to be addressing Bertolt behind her when she says that the Titan could prove to be an ally.
The Titan forms of all the known Titan Shifters bear at least some resemblance to their human form. The Dancing Titan has Ymir's black hair. The Female Titan shares Annie's Cleopatra Nose and blonde hair. The Armored Titan shares not only Reiner's hairstyle, but even his Heroic Build. The Colossal Titan is able to peek over the Wall and Bertolt is 6'3. Finally, the Rogue Titan has the same wild-eyed look as Eren. Interestingly, the Colossal Titan is the closest of them to having the Uncanny Valley disproportionate appearance typical of Titans. This may hint towards Bertolt's claims that he has "no will of his own" and therefore has less of a sense of self than the others.
Armin asks Jean and Reiner to cover their heads with their hoods before fighting the Female Titan so she won't kill them (in case one of them is Eren), but Reiner removes his hood again right before engaging her, as he obviously needs her to recognize him.
When Reiner is grabbed by the Female Titan, pay close attention and he turns his head and winks at her, in addition to carving the words into her palm. Plus he somehow survived being squashed, and there's steam briefly coming from his head while carrying Armin. And throughout the scene we keep hearing inner monologues from Armin and Jean, but never from Reiner.
You know that one image of the history of humanity, with the people fleeing the Titans? With what looks like runes? Turns out they're a Genius Bonus for anyone willing to decipher them. "People fleeing their homes due to the appearance of the Titans. Unable to match the overwhelming force of the Titans, humanity was forced to sail to a new world. At this time, most of humanity had been annihilated, but the majority of this was at the hands of their fellow humans. Those who gained passage aboard the ships were only a small fraction composed of the rich and powerful. The voyage was rough; about half [of the ships?] vanished without reaching the other shore. In the new world, great Walls had originally been prepared. This new continent, we shall revere as sacred. Within these Walls are the ideals of humanity. Within these Walls, let us create a world eternally without strife." Suddenly, so many things either make horrible sense or leave even more horrifying questions in their place.
Why didn't the Armored Titan ever show up at the Battle of Trost? Because Reiner decides to follow Jean's order to run to HQ rather than stick to the original plan — just look at Annie and Bert's hesitancy when he bolts. Also, they found out who's inside the Rogue Titan, making them to reconsider their decision.
The Walled City seems to have a serious problem with institutionalized classism, with the wealthiest and most talented being given the best and safest places to live, while everyone else is more or less expected to die to maintain the city. The people who originally found the Walls and founded the City inside it were the wealthiest people in the world at the time. When they had to set up a system of government, naturally they would build it around the idea of preserving their own status.
Bertolt makes a big deal about having wanted to join the Military Police Brigade, stating he would probably quit if he failed to rank. However, he ends up being one of several people to enlist in the Recon Corps, a decision he doesn't look too happy about. This was a result of Eren's powers derailing their original plan, which would have required him to be one of those given access to the Interior. Dot Pixis points out that the loss of Wall Rose would result in an even greater purge of the surviving population, and only those with influence or a position within the Military Police would be safe from it. It also would have given him access to Wall Sina without arousing suspicion. But since Reiner joins the Recon Corps in order to remain close to Eren and aid their eventual plan to kidnap him, Bertolt has little choice but to follow him.
If kidnapping Eren was the primary goal of Annie, Reiner, and Bertolt, then why didn't the others transform to help Annie accomplish that mission? Simple. Because their mission also required them to locate the Beast Titan, which had not yet shown himself and if Eren turned out to not be the "Coordinate", still having some of them undercover would help their search efforts.
More of a 'heh, double meaning' observation, but during the trial, Eren basically tells the court to leave Mikasa out of it (with their panicked suspicions that she might be a Titan, too) and that he might be a monster, but she's got nothing to do with it. While he obviously means 'monster' in the literal Titan sense, he's probably also acknowledging his (brought up at the trial as well) pitiless killing of Mikasa's kidnappers as a child.
Jean is extremely negative about humanity's chances against the Titans, and is very vocal about wanting to join the Military Police Brigade so that he can live a safe, comfortable life. Now remember that he's from the Trost District, which was safely within the interior until Wall Maria fell. Afterwards, it was flooded with refugees and became the new front line with everyone fully expecting it to be attacked by the Colossal Titan at any point. He went from being completely safe, to living with the constant fear of the Titans and probably witnessed the brutal toll fighting took on the Survey Corps. Little surprise he became desperate to escape that, and return to the safe life he once knew.
Why was Jean automatically trusted and included within the group carrying out the operating within Wall Sina? Because he'd already been cleared of suspicion due to being in the group told that Eren was on the left flank. Since the attack came from the right flank, he couldn't have been the guilty party.
It's a little weird that the regular Titans walk around, some of them with surprisingly well-kept haircuts and groomed facial hair. They don't age, at all, including hair and nail growth. It's also why they don't need to eat; their bodies don't consume resources to grow.
The Female Titan often kills in a highly sadistic manner. It could be that Annie is employing psychological warfare against the Survey Corps. She does not seem interested in killing for its own sake and apparently fights only when attacked. Her mission to find Eren is much more important to her; the flashy kills might serve as encouragement for the Survey Corps to leave her alone. Also, hopefully fewer of her classmates would force Annie to do something she would regret; certainly, some of them like Sasha and Connie could have their morale broken by hearing of or witnessing such a display.
Ymir comments that she lived 60 years as a normal Titan, with her consciousness trapped in a state of constant nightmare, unable to control her actions. ZoŽ was right! There ARE human beings inside of Titans and most of them probably aren't malicious and would stop eating humans if they could. Of course, whether or not she can contact them is another story, but now her treatment of Sonny and Bean is completely spot-on, if still futile.
Almost all the people that fight Titans are short and very slim. It is outright stated that a lot of people are unimpressed at Levi being The Ace and being so scrawny and short. But when you fight by flinging yourself through the air, swinging on ropes, and aiming at a very precise weak point with All-Or-Nothing attacks, being small and light is much better than outright physical strength per se. So it is veryJustified. Also Truth in Television, as any activity that focuses on speed and precision will result in a lean appearance. Gymnasts in particular are on the shorter side, as it's much easier to control their bodies and avoid injury while in close proximity to equipment.
Since Levi is a near-legendary figure, people probably have unrealistic expectations of him. Plus, he is much shorter than most of the cast; this probably surprises people more than his scrawniness.
Eren tells Levi that he's much more talkative now in the aftermath of his team's death. Levi says he's always talks this this much. Cue a panel showing that they're the only two in a large mess hall. This could imply that yeah, Levi isn't very talkative to Eren. They've only just met. When he's around his now annihilated team, he probably speaks much more, thus accenting how much more alone he's suddenly become.
It seems like it was too risky for Annie to kill the captured Titans. It wasn't likely that the new experiments were going to reveal anything. However, Eren was supposed to be involved in the experiments. Eren, who they already suspected of being the coordinate. In other words had the power to control Titans.
In retrospect, it's pretty obvious that humans didn't actually build the Walls. So, humans built these three massive Walls a hundred years ago before the Titans could eat them all, but when the Colossal Titan creates a hole, they have no way of sealing it? You do the math.
It may or may not be intentional, but when Annie spars with Eren in episode 4, there's a split second where she covers her neck with her arm.
Why does the otherwise aloof Annie seem so savvy about the contradictions of the society inside the Walls? Because she comes from somewhere outside the Walls, and wasn't brought up to consider it "normal".
If it was Reiner who mistakenly told Annie that Eren was at the right flank, was her harsh treatment of him when he tried to approach her Titan form a "punishment"?
Doubtful. More likely, she did that to help him keep his cover after he delivered a message. Squeezing him let him look like he had to fight her to escape, and let him get close enough to cut his info into her hand. That, and they both likely knew that they were going to have to guess at Eren's location, probably because Berthold and Reiner got different info themselves. Annie looked like she was anticipating a change of orders from one ore both of them; Reiner did take care to let her know he was the one coming for her.
After Titan!Eren manages to land a good punch against the Female Titan, she starts looking exhausted and slightly afraid of him, and we get a close up of her eyes (both manga and anime) as if she's thinking. Following that, is it possible that she intentionally tried to make Eren recognize who she was in order to distract him?
Notice that when Mikasa is chasing the Female Titan, Annie tries her best at escaping rather than attacking her no matter how much she's slicing her up (which, especially in the manga appears to have been a lot). She eventually throws a punch once, though it's seemingly to warn her off more than an attempt at harming her. When Levi whom Annie doesn't know appears however, she almost immediately lunges for him while still ignoring Mikasa. Armin wasn't the only fellow trainee Annie didn't want to kill.
Wall Sina, Goodbye gives us the revelation that Annie has nothing against humanity and considers her mission stupid/meaningless, but ruthlessly follows through with it none the less. It would likely explain why she refers to herself as a "weak girl" wishing "people who go with the flow to be considered human too", when that's exactly what she's doing; following a mission despite not believing in it, considering herself too mentally weak to defy it.
In a Meta sense, there is a reason Eren is the protagonist, besides his inspiring levels of determination, which is not lost on too many characters. Look back to when he met Mikasa, especially the aftermath when his father chews him out. Eren argues that 'rabid animals need to be put down, even if they happen to look human'. Besides the most obvious applications of this, the oddly humanoid Titans, this sentiment becomes more important the more we learn about the Titans and just why they look human in the first place. Then, it becomes even more important when humans that aren't even Titan Shifters become enemies. At Chapter 57, Jean speaks up, saying he probably wouldn't be able to kill a fellow human being, having pledged to fight for humanity. The next panel is Armin and Mikasa looking somewhat dumbfounded. Eren, Mikasa, and Armin, the Power Trio, have been looking out for each other and their friends, but when it comes to mankind, they have always been more about ending the threats and gaining foot holds than defending the cause.
Titans eating humans as an attempt to regain their human form could add another layer to their Our Zombies Are Different status, given the original myth of zombies "eating human brains" was out of a need to replace their missing one. Said revelation also explains why Titans don't eat other creatures.
Why do Titans respond to the call of Titan Shifters? Because they realize it isn't a normal titan and want to eat the shifter in an attempt to become human again.
There's a lot of discussion in the early part of the series about how the system is broken and places the most technically-skilled soldiers away from the front line where they won't get any experience or do anything useful; this is sadly dismissed as the result of corruption and stagnation. Or more likely it's entirely intentional, to provide a pool of expert operators for the more complex and probably much more difficult Anti-Human Gear, since the government is apparently far more interested in assassinating its own people than in fighting titans. Building on this, the reason most of the military police are allowed to become so incompetent is because these are the candidates who aren't quite corrupt enough to be willing to act as the government's pet killers. Their incompetence makes them that much less of a potential spanner for the real military police.
If you look closely, some of the less deformed Titans have facial features belonging to races that are not seen within the Walls. For example, the Titan that ate the soldier talking to Erwin during the Clash of the Titans arc is clearly Asian, and the Deviant Titan pulling on Historia and Nanaba's tree during the Female Titan arc is most likely Hispanic. Now that they are revealed to be transformed humans, it's possible that the majority of humanity wasn't eaten during the Titan-apocalypse...it was transformed.
"Ackerman" seems to be the odd man out as far as meaningful names in the series, but think about it. It means "farmer". As in "one who harvests". In other words: a reaper.
In Levi's story A Choice With No Regrets, he had a little sister figure in Isabel who ended up killed by Titans, upon which he found her severed head in the aftermath. It might explain why he seems to take Petra's death the hardest when his second group is killed; she reminded him of her.
In the above mentioned story, Levi eventually made a choice to leave his comrades behind which resulted in them getting killed. It probably explains why he's so understanding towards Eren when the latter blames himself for Levi's squad's death, having gone through a similar thought process himself.
Why do the family's of the Levi Squad not even consider that they died? Because the Levi Squad have survived several scout missions without casualties, even while their comrades were dropping like flies around them. The fact that the Scouts are returning so soon means the mission went badly, but the families of the Levi Squad have no reason to think that THEIR loved ones died.
Aside from the sequences in-story that foreshadow the identities of the Colossal Titan and Armored Titan, Isayama also instructed the animators to leave some pretty spoilerific hints in the second anime ending: Pay attention to the way the characters of the 104th Trainee Corps are grouped along the wall. Everyone in the second grouping of characters (with the exception of Krista, who has a different secret) are all titan shifters - what's more, their identities are revealed in the same order that the camera focuses on them onscreen. Eren is the only titan shifter not included in this grouping because he fights for humanity.
Eren unconsciously knew that biting his hand triggers a Titan transformation because he has memories inside of him from Frieda Reiss, the one possessing the Coordinate ability before him and who also transformed by biting her hand. Related to this is why he found Krista's personality to be false. A deeply buried part of him knew full well what she was actually like and was sending vague memos of cognitive dissonance and/or disapproval every time he clapped eyes on her. Once she drops the act and the alias, suddenly, he can cope. Funny, that. Also, did Eren become so protective of Historia because of those very memories?
While many have criticized the whole "political corruption" arc for taking focus away from the Titans, it actually fits in perfectly with the series' overarching theme: Humans Are the Real Monsters. The Titans and their origins prove this on a more literal level, while the Antipersonnel squad, the autocracy's willingness to abandon the people, Kenny's willingness to shoot Levi dead, and pretty much every other despicable person or act committed in and out of this arc complete the allegory by proving it on a metaphorical level.
Why is Rod Reiss's Titan form so ridiculously large? Kenny points out how impractical it is for what was supposed to be a "combat oriented" Titan. This is because it's a mutation—Titan fluid is meant to be injected into the bloodstream, not ingested. Notice that most of the mass is concentrated on Rod's Titan's stomach, where the fluid ended up.
The Reveal that the Beast Titan is a Shifter along with the existence of Titan serum handily explains how it was able to turn an entire village into Titans overnight. It may simply have snuck into the village while human and poured the Titan serum into the town's water supply. The Titans are deformed for the same reason as Rod's mutation — they ingested the serum instead of having it injected.
When Wall Maria fell, it's stated that 10,000 people or more were devoured. A year later, the government forced 250,000 refugees and soldiers to take part in an operation to reclaim lost territory. All but 100 died. All that death, because of two 11-year old children and a deluded 12-year old. All three are exactly what one might find in real life — Child Soldiers forced to commit atrocities and left to figure out how to cope with what they've done once they're old enough to understand it.
Reiner: We were just stupid kids that didn't understand anything.
Episode 9 notes that the attack on Trost was so devastating because the Survey Corps were out on an assignment at the time, leaving the most experienced soldiers elsewhere when the proverbial shit hit the fan. The timing couldn't have been worse. The infiltrators knew this, and probably used it to their advantage to maximize the death toll. Likewise, the attack in year 845 began just hours after the Survey Corps had returned from a devastating mission outside, from the south gate.
So since Titans don't have digestive organs, that means they likely don't have stomach acid either. Meaning that when a person is swallowed whole, the only way they'll die is either through dying from the heat or drowning in their comrades blood. It's clearly what would have been Eren's fate had he not turned out a Shifter.
Just how crazy was the Titan training that Annie went through as a child? She can remain in her Titan form for long stretches at a time, can run at high paces, take out dozens of soldiers regardless of how strategically they try to stop or kill her, maneuver herself through a horde of attack Titans without anyone noticing when her Titan body is eaten, transform a second time several minutes later while acting like she's not tired at all, focus her regenerating abilities on the part she should prioritize, crystallize her skin, etc. Compare that to Reiner and Berthold, who only had to kick or run through a Wall, and you're easily left with the impression of Annie having had much more severe training than them. Sure, her Titan strength partly stems from her human strength, but considering how masterfully she controls her Titan body, it still means she'd have had to spend a good lot of her childhood (when she was younger than 10) transforming into and learning to control it.
While it still doesn't say anything on her Titan training, Wall Sina, Goodbye does further explore the hellish training of which she went through as a child. Every day, her father would make her kick the padding on his legs, padded poles or sandbags hanging from trees from before the sun rose till it set in the evening. At one point, he scolded her for trying to take a break to which she lashed out on him as a result. And, while it may be looking too deeply into things, his reprimanding that caused her to snap is very similar to the one we hear in her dream in episode 23. It could be she woke up at that moment as she didn't want to recall her beating of him.
Thanks to the revelations from Chapter 46, everything concerning Reiner. He's been completely insane the whole time.
Nakedness is also a sign of purity, especially when it's a kid. The Titans are transformed humans, trapped in a nightmare. They have no idea what they're really doing, and there's a good chance that, if Annie, Reiner, and Bertholdt are any indication, most of them are kids.
In Chapter 47, Ymir reveals that she spent 60 years wandering around as a regular (mindless) Titan. It was only five years ago that she was able to wake up and regain her human form. She describes those decades as being trapped in an "endless nightmare", but doesn't recall exactly what she did during that time.....including having devoured Reiner and Bertolt's friend, Berrik. Bertolt states it was "the same" for them as well. If this is the norm for Titans, then that means all Titans were once human. And they all have the potential to wake up, and regain their human minds and forms. That means the only difference between Shifters like Eren and regular Titans is that they were able to escape their Lotus-Eater Machine. In the same chapter, Ymir says that "Eren doesn't remember either" which means either a) She is assuming Eren was a Titan from outside the Walls as well. Or b) Eren killed his dad after he injected him due to a partial or full transformation and reverted back into human form without remembering anything. Possibly c) both. Keep in mind, we still don't know where he has been during Eren's training.
In Episode 3, Jean says that he is from Trost. So during the Battle of Trost, on top of everything else, he's watching the Titans destroy his home town.
The Wall Cult has a lot of power and influence, and early on, Eren and company complain about what a pain it is when it comes to doing anything to improve defenses on the Walls, and how much trouble it was just to get the rail-mounted cannons put up there. The Wall Cult knows about the Titans in the Walls. They're desperate to protect the integrity of the Walls themselves because, at all costs, they must not be allowed to awaken.
Chapter 50 reveals the real reason why the other Titan Shifters went after Eren. He's a Coordinate, capable of controlling Titans. What about the Beast Titan? And if the abnormal Titans have seen this power before, who was the first?
Chapter 51 has several examples.
In-universe example when everyone realizes the true nature of the Titans. Hange and Erwin in particular look horrified as it sinks in that all this time they've been killing and experimenting on people.
When we see the photograph of Connie's parents, it turns out that the Titan Sasha fought at her village was Connie's father.
Combined with what Ymir said about being a Titan for 60 years, and the true nature of the Titans. How many ways can one become a Titan, and what can turn a regular Titan into a Titan Shifter?
The houses in Connie's village were all destroyed from the inside. If the position of Connie's mother is any indication, this is because the villagers were turned into Titans while still indoors. Since the Beast Titan would need to heavily damage the houses to get at the people inside, this means that either the Beast Titan is a Shifter, or it doesn't need to touch people to transform them. In the latter case, this means it was able to turn an entire village into Titans at almost the same time (otherwise people would have used the horses to get away) through a purely mental action. Imagine if it pulled that trick in a fight against human soldiers.
Word of God has said to base the story off his experiences as a victim of bullying. With that some characters and plot points have a more scarier interpretation. Certain Titans seem to be modeled off stock bullies. The Colossal Titan reflects the Barbaric Bully, usually a big and tough person who might have a Dark and Troubled Past, the Female Titan being the Alpha Bitch, the Armored Titan as the Jerk Jock, and Rogue Titan being the Bully Hunter... or more scarier: He Who Fights Bullies. Even more when you consider the whole Coordinate plot point. So the early, seemingly insignificant scene of our main trio dealing with a group of bullies was in fact symbolic for the entire series.
Isayama revealed the idea for Titans themselves came from an encounter he had with a drunk person. Basically, the Titans' concept of dominance and terror comes from bullies, while the lack of ability to communicate with them is inspired by intoxication.
When Levi and Hange begin their interrogation of Sanes, he is completely uninjured, but Levi is already covered in blood. He was not the first captured MP they'd tortured.
Eren's transformation into a Titan during the Battle of Trost did much more good than anyone realizes. The only reason the Armored Titan didn't destroy the inner gate like before, which would lead to the loss of Wall Rose, was because Reiner, Bertolt, and Annie held off the next stage of their attack to find out more about the Rogue Titan.
Based on Ymir experiences, she became a Titan Shifter by eating another Titan Shifter, if we go by that point, we know why the Titans are eating people. They are eating people in the chance of eating a Titan Shifter and returning back to normal. The eat everyone as they have no clue who is a Shifter and vomit their stomach so they can continue looking for a Titan Shifter. That just makes all Titans even more sad.
The antipersonnel 3D maneuvering gear is the perfect weapon to fight any human inside the Walls. Since firearms had no effect against the Titans, they're barely used and they haven't been improved in over a century. So, no one is prepared to counter or fight against the guns the Humanity Suppression task force uses. The Survey Corps are at an especially bad disadvantage because they're used to fighting giant opponents with swords and not dodging tiny projectiles fired at them by humans that they're fighting for and trying to protect. It was only because of Levi's sheer skill that he was able to dodge both of Captain Ackerman's shots while the rest of Hange's squad were taken out and killed via Boom Head Shot.
There's also the fact that Captain, or Kaney, Ackerman was a serial killer who took out over a hundred MPs over the course of a few decades and is considered a legend in the inner district. He was also the the man who Levi lived with when he was younger. It's no wonder Levi says he's an aberrant human; whatever Kaney Ackerman taught him when they lived together must have really messed him up one way or another.
Titans have no reproductive organs and yet they seem to be numberless. Now that we know that they are transformed humans, two horrifying possibilities can be considered: either the story of the rest of the human race being devoured is inaccurate and the Titans are the vanished humans. The second might be even worse—somewhere beyond the Walls, humans are still being actively hunted down and turned into Titans.
Marco's death is a pretty big deal, but consider what would have happened to him if he hadn't been killed: he'd have ended up in the Military Police, lumped in with the heartless bastards in their ranks. Black market weapons dealers, widespread corruption and greed, and just all around terrible people. Marco would have been bullied into complying with his superiors... or worse, used against the Scouting Legion, or killed for standing up to them.
On the Titans in the walls: the ancient Japanese practice of Hitobashira, or "human pillars," is human sacrifice to the gods by sealing human beings into the construction, in order to make the construction stronger and stable. If the gods were pleased, the construction would last longer. And those constructions are said to be haunted by the ghosts of those sealed within it...
Grisha Yaeger's Titan form looks eerily like the Beast Titan in size and shape.
If a Titan Shifter is only created after he/she was a mindless Titan eating another Shifter, then who did Reiner, Annie and Bertolt eat? (Though the existence of serums capable of directly transforming a person into a Shifter might be a possibility.) The fridge horror gets dampened a smidgen thanks to chapter 65. A Titan doesn't need to eat another Titan Shifter to become a shifter themselves, they simply need to ingest their spinal fluid. It's likely that the serums that Reiner, Annie, and Bertolt were given was a mixture of the standard Titanization serum, and another shifters spinal fluid. But this raised questions of why Rod Reiss is so intent on having Historia eat Eren...
If all it takes for a Titan to become a Shifter is for it to ingest a Shifter's spinal fluid, that means the cure has been in human hands this entire time. We don't know how much fluid it would take, but essentially, it means that once somebody gained the necessary advancements to perform a successful lumbar puncture on a Titan shifter and cram a dose of fluid down a Titan's throat, the problem could have been solved as quickly as the cure could be administered.
The Titan serum Rod was carrying with him comes in a small organized kit with a little case for the syringe, bottle and replacement needle. All in all, the thing looks industrially produced or at least manufactured for that specific purpose. Somewhere, somehow, Titan serum is being created, perhaps on an industrial scale.
The hated Titans are actually human beings who likely have no clue of the chaos they themselves are creating. Even worse when you imagine that the smaller, 3-4 meter Titans may be mutated children. That's right — our heroes very well might all be baby killers.
Annie, Reiner, and Bertolt essentially spent several years learning and mastering combat skills to kill themselves. They literally grew up watching humanity gutting themselves to ensure their own comfort while being told over and over again that Titans are a conscious, sentient race of giants who do things like "claim territory". It's uncertain how much they know about all of it, but they definitely know enough to realize that Titans are no more culpable for eating people than a two year old is culpable for swallowing a toy. And because their paths diverge so drastically, due to assignment and mental health issues, each one of them was completely and utterly alone.
The Titans often swallow people whole. Think about that for a second. Titans become Shifters by ingesting a Shifter's spinal fluid, not just eating them; this is why the Titan that ate Eren didn't become a Shifter. Eren was missing a couple of limbs and he was bleeding profusely, but his back wasn't injured at all. It's a slim chance, since the Shifter would have to be unconscious, but a Shifter could just drown in a Titan's stomach. God only knows how many Titans missed becoming human again by that much. How many Shifters' deaths were completely wasted the same way?