I'd probably throw in everything, in some cases splitting things off into alternate material planes or making similar to someone's idea for how the original DnD worlds and the MtG worlds interacted (I can't recall who posted it, but I liked the explanation as a reason why it was difficult to get from the MtG worlds and the DnD worlds).
For me, I'd probably set up spheres of worlds which divides them so that you end up with something like:
- MtG worlds (Ravnica, Theros, Dominaria, and the rest of them)
- DnD worlds (Greyhawk, Forgotten Realms, Dragonlance, Birthright, Eberron)
- DnD alternate worlds (Mystara, due to how different a lot of it is in regard to the planes and immortals, though maybe I'd just throw it in with the rest)
- DnD distant worlds (Dark Sun, due to it being cut off from the rest of the planes. Gith managed to get there somehow so maybe a spelljammer can reach it)
A campaign set in something like this could be quite fun, though due to how large it is it might miss some of what makes each setting unique and fun.
Some changes to the way things work in each setting could cause issues for some characters. I'm sure wizards from the forgotten realms would be a bit put out by their lack of power in birthright, not having a bloodline would mean that they are limited to divination and illusion magic and, I think, 1st level or lower spells of the other schools. If they made it to Dark Sun, they might not be able to cast magic at all until they learnt how to draw it from the life energy of the plants around them, and in dragonlance, they might be hunted down as renegades and made to join an order of high sorcery.
