So, I've currently got two ideas bouncing around and struggling for dominance. They're similar, but still have some substantial differences.
The first is Dungeon Keeper: The RTD. You're the dark lord of your own personal dungeon, and must attract (and improve) minions, build traps and rooms, battle the forces of light and good, and generally pursue whatever deranged schemes you like.
Everything is custom or customizable, however. For instance, while standard DK minions would likely exist, the players' starting source of lackeys is completely custom-built as part of character creation. Similarly, players would probably start with the absolute basics in terms of what rooms they could create (lair, hatchery, presumably library to research further types), everything else would require the players to actually sit down and say "I want to be able to make a workshop, and it'll do this and this." In some cases this would just be generic, vanilla types ("You've designed a workshop. You can build stuff in it."), while in other cases it'd show the Keeper in question's particular flair ("You've designed a butcher's workshop. You can build stuff in it, but especially things made out of formerly breathing materials.").
For spells, players would have either very, very broad, somewhat eclectic spheres or schools (similar to colors in Magic) from which they would then need to research individual, thematically appropriate spells (so if your sphere is "fire, destruction, chaos, damage," you probably can't research a healing spell).
Minions would generally obey your commands, but both be lazy and disloyal, and have their own agendas, mostly involving not dying and amassing great riches for no effort. More powerful, competent creatures would generally have more ambitious goals and desires. Minions could level up through training or combat, and gain or lose loyalty depending on how sweet a deal they think serving you is.
The overarching goal, such as it is, would be to amass power and wealth and pursue whatever other vile schemes you can come up with. Searching for powerful artifacts or their fragments is always a good one. Players might keep one permanent "home" dungeon, but otherwise it'd be entirely possible to abandon or create new ones, usually to build up your power base enough to accomplish your goals in a far-off area.
The second is somewhat harder to explain, but "Roll to be a Power Rangers Villain" comes somewhat close, only very high fantasy and concerned with politics. Players serve a dark lord, probably the unreasonable kind with no tolerance for failure of any kind, and one with a fairly wide selection of scheming, competing minions.
Players would have holdings, mostly consisting of their minions at first, and would be able to amass (or more rarely, lose) more by doing well, either by succeeding at deliberate schemes to increase them (robbing a village, gaining a tribe's fealty), or as rewards for victorious service. A player's holdings could determine any number of things, but the most obvious would be status and what you're able to do (without a smithy it's kind of hard to custom-equip your goons, for instance).
Most of the player's actions would revolve around dispatching minions to do their bidding; players could take a personal hand in affairs, but doing so might be dangerous and would probably have some limitations or downsides to it. No one specific enemy would necessarily oppose them, but this world is high high fantasy- attacking some nameless, out of the way village and encountering a flame-throwing martial arts monk would not really be "odd" in the traditional sense. Of course, the players' actions are likely to attract unwanted attention and ire, so the players should have plenty of reliable, well-organized enemies before too long.
Politics would be very important, so in addition to your personal schemes and the commands of your overlord, the requests and warnings of potential friends and enemies would be a significant source of objectives. Properly appeasing your "fellows" could have benefits ranging from a loyal ally to not being openly attacked just yet (though, the overlord obviously forbids most overt aggression, so said enemy better know what they're doing in that case). Displeasing them could, of course, incur all sorts of unpleasant (if sometimes petty) consequences.
I've also considered combining them somewhat, ie you're Keepers subservient to a greater power, but I'm not sure.