I loved the ask about the Volturi (but actually Bella) solving the Kira case! In your answer you talked about how the Death Note and Twilight universes are compatible because the Shinigami don't contradict the preestablished rules of the Twilight universe, and I was wondering what other paranormal/fantasy media you think could be crossed over with Twilight without altering the rules of either universe?
What's Already Been Discussed
Anon's referring to this post. Specifically the first part where I ramble that Shinigami could exist in the Twilight universe and the Volturi in the Death Note universe and both worlds would remain relatively entact.
I have also infamously argued that Harry Potter is a bust, either the wizarding world exists or the Volturi exist, you cannot have both (and my money's on the Twilight vampires).
The demons of The Exorcist I think would work provided they're not common (as they don't seem to be)
The MCU and Pokemon work with some minor changes to the respective universes but are on the whole fairly entact. However, X-Men don't really blend well.
Rather than go through all the media I know, I think it's easier to break down some ground rules of when this occurs and when it does not.
For a clean crossover we have to look at where the power lies and how that's shaped the respective universe.
The Twilight universe hinges upon the existence of the Volturi. Without the secret, the world as we know it would be very different. We'd be slaves of tyrannical demon kings if not already extinct with vampires eating each other in starvation.
This is not a self-enforcing law of the universe, there's a reason it exists, and clear consequences for those who dare defy it. In other words, for Twilight to be at all similar to what we see in canon we need a) a governing body ensuring the vampires are kept in check and kept secret b) a reason for this governing body to exist.
If there are humans or else creatures populous enough and strong enough to defend themselves, then there's no reason the Volturi would ever have formed, they'd leave defending mankind to these other beings.
Then there's the other issue, if these people are humans, if they're disorganized (or at any point were disorganized/could be made disorganized) it's highly likely they'll simply be turned into vampires and whatever society we would have seen them create (i.e. Harry Potter's wizarding world) simply never comes into existence.
If there's another set of creatures that stalk mankind and devour them then they either must be able and willing to keep the secret themselves, be wiped out by the Volturi, or be at active war with the Volturi.
If you can check all these boxes then your worlds can very easily coexist. If you can only check some then the mesh of the worlds may not have a Volturi, your new people may be under the rule of the Volturi as well, etc. If you can't check many of the boxes it's probably best for a new thought experiment.
i.e. for Harry Potter, without the wizarding world there's no standard education, just very gifted humans who are likely turned very quickly. There's no Hogwarts, no Diagon Alley, no wizard dark lords, and at that point what do you even have left of Harry Potter? For most people, that's not worth a crossover at all as what they really wanted was Edward Cullen attending Hogwarts.
Some More Examples that Do Work Off the Top of My Head
The Revolutionary Girl Utena
Anthy herself is extremely powerful, something no longer quite human, but her world and her gift is very insular. She's a one-off event and something that could have escaped vampire notice.
Star Wars takes place eons ago in a distant galaxy. It has an expansive set of worlds. More, the Force would be something that would make vampirism a thing. The Star Wars world itself would be very different and perhaps face its annihilation as vampirism spreads, but it wouldn't collapse under the weight of Twilight.
We see humans who perhaps have the capability of being gifted summoned to another dimension which does not have vampirism. However, it's entirely possible that the Tokyo Hikaru, Fuu, and Umi are from belongs to the Twilight universe and that they themselves have latent gifts that would have been discovered were they ever turned.
Sakura and her brother Touya are extremely gifted humans, stupidly so, but through them we see little to no organization among magical users. There's a craft passed down, yes, and Clow Reed's surviving family is a prime example, but it's not as organized as the world of Harry Potter.
More, the cards are only one facet of magic, with many different techniques of how to access it.
Sakura, in her future, may have to look over her shoulder for the Volturi to come a knocking.
Puella Magi Madoka Magica
Now, the Incubators have a deep hold on mankind and could (and do in some timelines) enable the destruction of mankind if not the world. However, their technology is extremely advanced and their appearances relatively infrequent and innocuous.
Vampires could be a consequence of a wish and regardless are unlikely to be able to see witches and labyrinths. The Incubator bodies are disposable, and killing them would accomplish nothing, and the best the Volturi could hope for is to try to waylay the Incubators before they can make contracts if they're even aware of the Incubators at all.
However, in this case, I think the Incubators win. They would eliminate any threat to their operation before it could organize itself and destroying Twilight vampires is within their realm of capabilities.
Likely, instead, the Volturi would be allowed to organize themselves and keep their own secret and would never be made aware of the magical girl ecosystem that plagues their world.
John Coeffy is a singular phenomenon and a very gifted human. He easily fits into the Twilight universe.
This one... almost works but not quite. Elsa herself is a one-off, extremely gifted, human. However, she would be turned by the Volturi probably during the events of Frozen itself.
As coronated queen/crown princess, Elsa's a very public figure who then very publicly displays her power in front of the entire kingdom. Aro would hear about her, Demetri would easily be able to find her, and Elsa would then be turned and inducted into the Volturi.
Anna is also likely turned in case the bloodline carries strong gifts (whether Anna herself would be gifted or not is up in the air).
Similar to the above. Rapunzel after having been found, is relatively known to have her magic hair and healing (and later, per the TV show, dangerous indestructible hair). I imagine she'd be too tempting of a target and would be turned by the Volturi as soon as it's convenient for her to disappear (and given her many adventures this would be sooner rather than later).
