Campaign setting crossovers.

[Fortinbro]

So me and my friends have three games going currently, alternating with each other. The three DMs, myself included, have all thought it would be an amusing idea to set up temporary crossovers. Each game is its own multiverse, so this would have to be through some exceptional dimensional shenanigans.

I was wondering if anyone else has done something like this before and if they have any tips for doing it?

[Falcon X]

Awesome idea. First place I would look is the old 2nd Edition Planescape Setting. In it, there is a Multiverse where each Universe is held together by the Astral Plane. All Universes link up to the Outer Planes (Realms of the gods).

Some ideas:
A. You could also have one of the Universes be a Pocket Dimension of another. Perhaps the pocket dimension was created and is manipulated by the Bad Guy in another universe. Thus, you all are actually fighting the same person.

B. Just have them find a portal. But when they get to the other side, we realize that the physics in the two worlds are different. For example, the people in one universe are twice the size of those in the other and are giants in this world, or you have a Planet Krypton effect and you all become Superman.

C. Recruit them into an inter-dimensional mercenary guild, or postal service. We did this once. Our party was in a different dimension every game. One of them was filled with cats...

D. Know that when you get to a new Universe, the gods that reign over it may be different. Your cleric might find his job and powers dramatically change. Maybe he will try to start a church in this universe and that is your new quest. Maybe he tries to convert another party.