I've always loved the idea of the Hollow World setting that TSR put out about 20 years ago (and it's various literary inspirations, like Pellucidar). In particular I liked the idea that the Immortals saved cultures on the brink of extinction by moving them to another place where they can thrive, in whatever relative fashion. Then I thought, what if the cultures were literary archetypes or even specific characters or places?
What this idea has done is give me another idea, a moment of inspiration, after reading a little about the Appendix N kickstarter that's going on for DCC. I've always wanted to run a really gonzo campaign with whatever system suits my fancy (and for this one, it's probably going to be Pathfinder, since I have so many sourcebooks for the players to snatch from). I call it The Repository, or The Library. It's where all fantasy literary ideas go after they're published for so long and forgotten about, or only cared for by a segment of existing readers (take, for instance, various cult Appendix N titles). The "world" would be populated by characters, places, cultures, etc., that exist within fantasy fiction - all fantasy fiction, whether set in the modern day or in some forgotten time. Think of it like looking in on a sort of "Neverending Story" of fantasy literature. It's the supreme fantasy mash-up. You could have Conan-esque barbarians alongside elves alongside technomancers alongside psi-warriors alongside evil Cthuloid cultists alongside Arthurian faeries alongside mythological heroes alongside kobolds alongside Harry Dresden alongside Buffy alongside.... well, you get the picture. Any idea, literally any idea, made possible by the various permutations of the D20 System and its plethora of genre sourcebooks and settings, will be possible, including monster classes. It will be a campaign limited only by the sheer imagination of those involved.
Sure, there will be imbalance. We'll handle that as it comes. Sure, it creates too many options. That's part of the fun.
Think about it - how fun would it be to mix a class like the Psion from the Complete Psionics Handbook (3.5E) with a cool prestige class from D20 Modern or even... choke... D20 Star Wars or D20 Fading Suns? Yes, I think this could be a great idea. How would Pathfinder's Gunslinger work with a similar shoot-em-up style of class from the various 3rd-party Modern supplements? Let's go one step further. How about using the Troll monster class from Savage Species and combine it with Fast Hero from D20 Modern or something equally offbeat? What about an aasimar with Jedi as the base class, who later takes levels in the Horizon Walker prestige class or learns spells like a Sorceror? I just sold myself on this with that one!
Oh yes... This will require some house rules. But imagine the pure insanity. Dice flying everywhere!