Fortunately the cosmos we have outlined thus far is sufficiently diverse to include both planetary romance locales and more avant garde possibilities. To tie this sliver of the conversation back into the larger issue of campaign setting diversity, one option between the extremes of too few or too many is to create a single campaign world capable of supporting a variety of thematic tastes and campaign styles.
The problem I see with that is with one world, if the playstyles and thematic tastes are say on different continents, then you are missing large parts of the world, if they are connected as such for all. Travel to those other continents would mean switching the playstyle or thematic taste you prefer so are confined to jsut the continent that houses your style/tastes.
Now, if you took a world, and mapped it out for people and made sure they KNOW that anyplace on this map/globe/atlas can be altered for yourself, and then use that world and plug into it various societies then it MIGHT work. So that everyone has a world to play on, but can take those thematic tastes and playstyles across the different "dimensions" of that world.
You probably already understand what I am saying, but just in case, and for others...
Take a round planet(oid), give it its needed moon(s), the water and land, set some cities up, some kingdoms, and some "dungeons".
With this world you would develop the equivalent of a DragonLance setting, Forgotten Realms, Greyhawk, all using the same world.
One world book with all the places on it as basically your games atlas. Then fluff books for the different settings so your setting C would have the city of Blahhaven, and settings D and H would have it in the same place.
Using your core set of monsters and sucj you fill it normally, but just have your factions, deities, NPCs for the individual worlds, but keep them small enough for gaming purposes, not expanding them into bloated canon for novels. Have enough places on the world that people could travel all over, and you can have adventures (paths) all over the place that don't have to connect, but could easily move from one to the other. Have the settings as a NOW, not a big past and future that will be detailed...just adventures.
It would pretty much be, in D&D terms, Toril that had different settings in different locations but the entire world would be one setting, and the world would be re-used for other settings as it were.
If it looks too closely like you are making Toril with Kara-Tur, Faerun, Maztica, and the rest, then it make instantly feel the backlash from all the Realms canon, and realms shaking events, since it resembles it.
REusing the same planet and locales for the different thematic tastes and playstyles though means no matter what setting you are playing in, you can relate to where others are playing, and the players would be able to bring them and merge things better should they want to mix-and-match settings as the worlds they come from would be the same for the most part, at least topographically.
This gives a large area to play around in for designing game material,a dn for players to make their own, and when they find an adventure they like they can travel there on the world and use it in any of the settings.
Except Bill's, his is
B.S.[MENTION=17465]Wizard[/MENTION]s.com
Are you B.S.-ing us?