I guess it's been a Secret Wars kind of week, so no reason to stop now...
If there were a Marvel Super-Heroes RPG in print right now, that would make an amazing sourcebook/campaign setting.
But since there isn't, what can the enterprising GM learn from this patchwork set-up. Obviously, there are games that are a little like this already - Torg, Rifts, and AD&D's Ravenloft setting all have some of that quality. For GameMasters like myself who have way too many RPGs on their shelves, it could also be a neat multi-genre/setting premise. The Multiverse has been destroyed in what has been called the "Critical Failure". A God or Gods have "saved" pieces of it and reconfigured them into a single world. Your heroes can come from any "domain", travel between them, or take part in local politics, depending on your bent. Things to remember about the Battleworld set-up:
-It has a single unifying ruler that may or may not be malevolent, but each domain has its own power structure. Who or what that entity is depends on the focus you want to give your campaign.
-There are also unifying elements like the Thor Corps that lets that ruler show his/her hand. Lands open to interact with one another, and lands normally closed off and independent/dangerous/wastelands.
-Several versions of the same characters exist in different domains, which would allow you to round robin between domains with a minimum of fuss. Say someone is playing a Grey Mouser type, that "thief" can essentially exist in different lands, and its player can play all versions (or for fun, have character sheets rotated around the table to really create alt-versions).
So for example, say I'm using Battleworld's map and just changing the names of the countries, what could we put in there? A few ideas (I hope you'll suggest more in the comments):
-One's favorite Forgotten Realm (or two; D&D)
-Alpha Complex (Paranoia) hidden under Communist Wasteland
-Arkham (Call of Cthulhu)
-Al Amarja (the island from Over the Edge)
-Languedoc (the Mythic Europe of Ars Magica)
And so on (your favorite Torg Realms, GURPS and Savage Worlds settings, superhero universes, etc.)
One of the advantages of using a Battleworld set-up is that you can try a new game in a one-shot (Paranoia seems built for this), and still make the players see it as something that "counts" in the larger tapestry of their gaming.
Now, there are games that don't quite fit this format. Spaceships and time machines would seem to go right off the map, and that map is meant to represent all of existence. There's simply nowhere to go if you have those means of travel. But there are ways...
Battle Galaxy: What if whatever force saved the universe reconfigured it not as a planet, but as a solar system, or small cluster of systems? That would allow your spaceship to visit whole planets based on other games, some planet-bound (Middle-Earth, Gamma World), some advanced and in possession of their own ships (the Kafers from 2300 A.D., Spelljammer).
Timeworld: Really love time travel? What if your Battleworld was only made up of historical eras? Instead of using a machine, you just walk through history. Perhaps that's the very nature of a Player Character, that he or she has the rare ability to walk between domains. And while that's fine if you're only in it to visit the past (and with no chance of paradox, I'm thinking everyone wants to go kill Hitler), you can also throw a big dollop of gonzo on top of it by saying the domains ARE historically linked. If you change something in the neighboring country, you could end up changing your own! "Guys, did the skyline on the horizon just change?! Crap!" It might even be an interesting experiment to draw the map so that North and South are Future and Past, with eras that really do lead one into the other more or less contiguous (for example whatever represents American history only touches European in the Colonial Meridian).
You know, I've got to stop coming up with cool campaigns and start running them! Off to my game collection!