[quote=VitaVitaAR]
... The come from different universes. Magic functions differently in different universes. It's not some homogeneous force across all of them. It's like how people have different blood-types, they can't use blood from a person with a different blood type but nothing stops them from interacting with it.[/quote]

It's almost as if we were on the same wave-length...

[quote]
I'd assume no-one will decide their character explodes when they touch magic from another universe.

Nothing like this is ever an issue in any crossover RP I've ever seen, it won't be an issue in this one.
[/quote]

But then this.

They may not explode. Or they may not have any effect. Or their magic is just a light-show. The important part is that outside of their universe their cut off from the source of their power and now in a new universe with new natural rules. They most likely can not draw from their old rules and there's something in the new universe that commands them.

Now, perhaps in this higher plane there's something that can allow them to use their power. In a literary sense this may create an incentive for people to act more since it comes with a reward of being connected to this as-of-yet-unknown-plot device. And with there being so many unaffiliated characters it'd be an extra material weight for when they feel naked without magic missile as they know it.

This also allows for a sense of balance between abilities. No one has the useful loopholes writers eventually write into their own magical systems so the hero can defeat the villain at the end. This creates a mechanical balance and keeps already wild abilities in check.

And no one may have ever brought it up because no one stopped to think about how disastrous someone joining as a character with infinite mind-reading potential can be and who can summon black-holes on a whim without any thought of how this happens in their universe in the first place and if it's compatible. Or folk are too polite and afraid of offering out feedback, as I've seen here. There's a lot of confusion about giving criticism with being rewd.