[quote=VitaVitaAR]
@Dinh AaronMK: I'm not particularly certain that it will be an issue given I see no reason for rules from one setting to somehow overrule others in any capacity. Just because characters originate from different settings with different rules does not mean that they cannot interact within a crossover RP. I see no particular reason that, for example, Saber's magic resistance and vulnerability to magical weapons cannot apply to all settings across the board. I expect a magic sword to hurt her but magical lightning to have no effect.
[/quote]

This is a small point in the entire issue, not all of it.

First and foremost it's the nature of magic that can end up conflicting. Questions like, "Where does magic come from" demands consistency or else you end up with a wild tied-up string of nonsense that leads no where and just leads to the inevitable "I have developed a game-over spell".

In some universes magic is innate in a person and can be brought out in training, as often seems the case in things like Elder Scrolls. Where as magic can only be born into a person as in Harry Potter or The Witcher as per genetics passed down or mutation. And in other rules magic can only be given through use of items, or is made incredibly easy as in Runescape.

These things are not necessarily compatible and leads to issues when they meet. It's in the same sense as someone crossing into another dimension where the physics are different. You may burn when you touch water. So what's to stop someone from exploding if they touch a trace amount of someone else's magic? They could go on a sub-atomic principle with universe-unique particles lending "magic" or it's more spiritual with the dead; though then if you're summoning the dead what's to say you can actually call them if you're in technically a higher plane of existence as them, that being the multiverse?

Or is everyone already dead and all fights will end up being like Sisyphus' bolder where they effort to finish the fight but can't?