In the beginning montage of the participants, Tengan is shown striking an action pose, hinting at the later revelation to him being a tough old man.
Each one of the death images demonstrates how each participant will be killed. Though not in the way you might think - they're symbolic representations of the characters' deaths.
Aoi Asahina's death image shows her stabbed with a knife. Guess what happens to her in Episode 2? Or rather not, since it turns out to be nothing more than a toy knife and some tomato sauce. The real knife ended up in the actual victim noted below.
Daisaku Bandai's death image shows him grabbing his chest. He would later do the same action while dying from poison.
The Great Gozu's death image shows his corpse being hung from chains. As the real victim of Episode 2, we see him hanging from the ceiling, tied up in electrical wires, and he's the one who got stabbed with a real knife.
Juzo Sakakura's death image shows him being cut by a katana, foreshadowing Munakata's betrayal. Juzo being cut in half also refers to his death by blood loss, caused when he cut off a part of his body (his arm) to get rid of the bracelet.
Miaya Gekkougahara's death image is being vertically split in half. Ultimately, she was "killed" in the exact same manner as her death image.
Kyoko Kirigiri's death image is her being hung by a rope, a common method of suicide. One can argue that by violating her NG code by refusing to kill Makoto before the 4th round she technically committed suicide.
Sonosuke Izayoi's death image is him being burned while tied to a cross, a combination of Joan of Arc's and Jesus's deaths - both known martyrs. Letting himself die so his girlfriend can live makes Izayoi a pretty big martyr.
Kazuo Tengan's death image is him being electrocuted by live wires. He was strangled by his many convictions to different parties and ultimately dies because of them.
Ryota Mitarai's death image is him being hit by arrows, the same arrows as the ones from Tengan's chuusen. Sure enough, Tengan completely and utterly fucks Mitarai over.
Tons towards Gekkougahara being Monaka's robotic avatar.
Her design is very, very similar to Monaka's. Both have the same distinct hair clumps, both sit in wheelchairs and the Scarf of Asskicking even mimics the ribbon on Monaka's DRAE outfit.
When the voice actors were being revealed, Gekkougahara was revealed with Asahina, Hagakure and Monomi. All three of those are returning characters, hinting that she was someone fans already knew.
Despite being purportedly a Therapist, Gekkougahara is completely mute and seems to have a massive expertise in electronics. Monaka was a well-established robotics expert
When everyone is taking votes on who the traitor could be Kizakura casually points at Gekkougahara on a hunch. Keep in mind that he's described to have a good intuition, and he's a talent scout, so it's his job to figure out when people have talents... Like deception.
Gekkougahara's death image shows absolutely no blood and she never once blinks. Both of these make a lot more sense when you know she's a robot.
Gekkougahara claims that her Forbidden Action was to turn right, but when told to escape with Naegi she turns right, and yet she's somehow unharmed.
Notice how, despite Monokuma saying the Final Killing is being broadcast, there isn't a single camera in plain view? That's because, as Episode 6 reveals, that fact is a complete and utter lie.
Despite Munakata's hypothesis that the mastermind's ultimate plan requires Naegi alive, Monaka mentions that she's helping Naegi and Asahina because they amuse her. Turns out that Monaka isn't the Big Bad.
Three of the NG Actions revealed in Episode 8 were heavily hinted at by prior episodes; Sakakura, despite his talent, is never shown punching anyone (using either weapons or his legs), Izayoi throughout the killing game kept rejecting Andou's sweets (despite unquestionably eating them in every other instance), and Kizakura is always shown with his left hand in his pocket.
Kyoko appears horrified when she checks her bracelet in Episode 2. That's because it's "passing the fourth time limit with Makoto Naegi alive".
Kyoko noticing the lack of the dent in the wall left by Izayoi and avoiding using the Secret Exit hinted at the secret behind the game stage.
There were actually plenty of hints to the identity of the attacker, or attackers, before the reveal in Episode 11.
The most significant one being the brightly lit monitors next to the body of each victim.
During the case of the second victim Gozu was closer to the monitor than Naegi, Asahina, and Gekkougahara.
The closed-room cases of the second and fourth victims, Gozu and Ruruka, especially since no one was near Ruruka during the time of her death.
In episode 7, Monaca states that she didn't know who the mastermind was, even though she would have likely seen the attacker, due to Gekkougahara being active during the sleep cycles, indicating that the attacker and the mastermind were not one in the same.
In Episode 3, Tengan is rather lax when they wake up after the second sleeping time, which Kyoko notes. This foreshadows that he knows the mechanics of the game.
Likewise, the fact that Tengan already seemed to know Sakakura's NG Code.
After Kizakura figures out Sakakura's Forbidden Action ("attacking someone barehanded"), he observes that the game would be extremely one-sided if Sakakura didn't have his Restraining Bolt. The same applies to one other key participant: Mitarai, whose Brainwashing Video would allow him to instantly pacify everyone if he was allowed to use it.
When the group discuss Tengan's dying message, Mitarai stares at his phone and starts muttering. It later turns out that the "hope" Tengan was referring to was the brainwashing program on Mitarai's phone.
Despair Arc:
In every shot of the classroom, there is always one seat left unfilled. Episode 5 reveals that seat belongs to Mitarai, who is staying couped up in his room working while the Ultimate Imposter uses his identity.
Juzo, irritated that Chisa stopped him from beating up Hinata, says that she's becoming too absorbed in her false identity as a teacher. Several episodes later, she becomes the mask.
The reveal in Side:Hope that AI Chiaki in her role as observer was made by combining everyone's memories was foreshadowed by the ending credits; the Chiaki made from memories is holding a Hope Fragment crystal.
Hope Arc:
Towards Kyoko's survival:
Compared to prior instances of the NG Codes being violated, the damage on her face is far less severe.
One can visibly spot Kimura's Cure W bottle on the floor in the scene where she "dies".
When her body is discovered, the version of the body discovery music that played during Asahina's Fake Kill Scare plays.
At the end of Episode 12 of Side:Future, during the montage of dead Future Foundation members, Kirigiri isn't among them.
The OP of Side:Future Episode 10, the episode immediately after Kirigiri's "death", states that there are 6 remaining survivors. If Kirigiri was actually dead at the time, it would've been stated that there were only 5 (Makoto, Aoi, Mitarai, Munakata and Sakakura).
. It's revealed that the "reflection" is in fact an actual building located below the original building, under the sea.
