The Jockstrap equipment is male-exclusive, and it protects against OHKOs... yes, a Groin Attack is a One-Hit Kill!
The She-Zombies found from Aswan onwards are an oddity when it comes to Jojo Zombies. They can walk in sunlight under which most zombies crumble beneath and most zombie encounters are within shaded areas (they literally never walk past the shade). So how do they bear the sunlight? The answer is actually extremely simple: they're all wearing niqabs and abayas, a type of islamic feminine clothing that covers the female's entire face save the eyes (niqab) and that completely covers the female's figure from head to toe (abaya) respectively. Because they're completely covered from sunlight, this is what allows them to move in direct daylight! Not to mention it's good as a surprise attack in universe, as Joseph being the only one who's dealt with Zombies and Vampires before would only expect them to attack within shade or during the night.
This also makes sense for Vins' choice of wardrobe, as it lets her move about in the sunlight.
Why are there so many cameos? Because Vins' is starting to lose track of what happened when; in her words, like a Phenakistiscope.. Because her observation is off; that is becoming the reality. All the Jojo series are happening simultaneously.
The Specials Stand is made up of 6 Stands, each having their own personality. They're all also somewhat representative of the Stardust Crusaders, even if it's not visible at first. How this is fitting is both in their appearance and personalities. As each one of the Specials looks generically the same, one could say that they're basically generalizations of each of their representative Stardust Crusader.
Specials 1 is a calm and collected martial artist, which fits for Jotaro who is the master of Poker Face and whose stand is focused on blunt force.
Specials 2 is a nihilistic gunman, which fits for Kakyoin who was lonely for most of his life because he couldn't relate to people who couldn't see his stand. His stand is also a long-range stand that shoots projectiles.
Specials 3 is a peppy knife user. Polnareff is Hot-Blooded person with a stand that wields a sword to match.
Specials 4 is a Gentle Giant. Even though Joseph in his younger years was more of the trickster type, in his older years he seems to show more care for his family. Such as his Freak Out when Holly was shown to wield a stand, ensuring the other Stardust Crusaders were taken care of by the Speedwagon Foundation and his interactions with Josuke seem to show he's very much caring at heart.
Specials 5 being a young lady who likes to play pranks. This references a few things about Mohammed Avdol, namely that he survives getting shot and later shows up alive, surprising Cameo and supposedly the other bits of DIO's forces. The act of playing as his "father" when the Crusaders finally come to get him is also a part of this. The factor that Specials 5 is a Lady could also reference his namesake, being Paula Abdul.
Finally, Specials 6 is a timid coward who fights with traps. While not exactly timid (moreso lazy really), Iggy certainly fits the bill. Especially with how he attempts to fight against Pet Shop and Vanilla Ice (using sand clones).
The requirement for Kakyoin to betray the Joestars being higher than Polnareff makes more sense when you take into account of his lonely childhood and trauma from his encounter with DIO, needing for you to gain his genuine trust so it becomes much easier into manipulate him.
Why is Jotaro the only one who is unsurprised when the PC turns traitor during the Betrayal Ending? Because he "can spot evil when he sees it".
The Dream World's Shadowy Figure. While it may seem like a simple easter egg towards Shin Megami Tensei, let's also think about what possible connotations it has with works by Carl Jung, and another popular series made by Atlus known as Persona. Carl Jung had theorized the idea that parts of the personality that one rejects or is ignorant of makes up the Shadow, and that it can appear within dreams or visions in various forms. In Persona, the Shadows are played out as such, and are often absolutely negative and monstrous parts of a person's personality. Given that the player character is the only one within the Dream World...what if the Shadowy Figure is the PC's own shadow? Also taking into consideration that it shows the hyper-realistic eyes that only show up in the "Mad World" Ending, that DIO commented that were the eyes of "someone born of pure evil"...what does this mean for the PC's capacity for evil, and how much it's suppressed?
Out of all of the Stands, Miracles is one of the few that is seen as weak by most people's standards in game. However, consider its ability: "The ability to mess with the subconscious". In game, this ability gives a wide range of attacks and status effects such as making people hallucinate pain to the point it becomes real, throwing up a Perception Filter, destroying people's senses, etc, making it rather good at hitting different elements or throwing status effects, and especially making people miss because they can't sense you. In reality, the ability to control the subconscious is a whole 'nother level of terrifying. The Subconscious is something that covers both a person's dreams, feelings, how we perceive things and emotions, as well as the autonomic nervous system which maintains plenty of bodily functions we don't control consciously, like our heart and breathing rates, arousal, body temperature control, etc. Given how Jojo is notorious for Heart Is an Awesome Power, Miracles is a stand that would be absolutely terrifying because it can effectively control everything that keeps you alive and makes up who you are.
The factor that Diavolo appears here and is a Recurring Boss in game from the second playthrough and up. Consider this: This game happened because Pucci used Made in Heaven to accelerate time, and Vins being outside of that makes the world into a huge jumbled up mess of converging timelines and whatnot that can only be set right by Vins death. In a sense, this is an entirely new timeline, and yet Diavolo is still dying his many deaths. This basically means that not even starting a new timeline will end Gold Experience Requiem's effects on Diavolo, and they'll continue unchanged!
There's also the factor that Kars appears in the Sanctuary as an Optional Boss once you get to Cairo. Think about it: the Ultimate Lifeform Kars is in the Sanctuary, and if he manages to exit it back onto earth, it's essentially the end of humanity because nobody could stop him now.
J.Geil and Joey are not the epitome of mental health, the former being a Serial Killer and a rapist targeting schoolgirls and the latter using creepy dolls as his Stand. But facing them as a female protagonist... Losing to them could very well mean you would be treated like Sherry or turned into a Murderdoll (Joey explicitely says he doesn't want to harm your "pretty face" when facing him in Singapore).
All party members have a cheap and somewhat reliable revive command called Cardiac Massage in reference to when Jotaro restarted his and later Joseph's hearts by gripping it and squeezing it so that their blood started flowing again. This quickly becomes Fridge Logic when even non-humanoid Stands (Such as Mr. Big) and otherwise normal humans (such as Speedwagon) have this command too.
To be fair, what Jotaro did to restart his and Joseph's hearts was a more direct method of CPR, in which part of it is focused on manually forcing a non-beating heart to start pumping blood back through the system. Given how simple CPR is to learn, it's reasonable for anyone with non-humanoid stands and otherwise normal humans can perform it.
It should also be noted that while everyone learns Cardiac Massage, Jotaro specifically learns Revive later on, which is a stronger version of it. It can be assumed that this is actually what he used to restart hearts.
Pretty much all stands have a special power of some kind, so some may find it odd that Red Garland's "power" is to punch really hard. However, one person put forward the theory that Red Garland doesn't just have the ability to punch physically, but the ability to punch their mind/soul. This is supported by the fact that Red Garland's attacks (even normal ones) are able to inflict a multitude of status effects on the opponent just by "punching them". It also helps that Red Garland's description states "Its punches can knock people silly" and the existence of Red Garland Requiem.
Word of God basically spells out that Red Garland's true power is to directly impact the soul itself.
. Because her observation is off; that is becoming the reality. All the Jojo series are happening simultaneously.