Hmmm, not sure if I can write horror up very well, but I could give it a go. Personally I think my strong suit is romance writing, but I don't think there is much of a calling for that in RTD land. Anywho your job is to sell me on the idea and make me want to DM it more than the other guy's idea. I'm hoping for some ideas I haven't thought of, but we'll see I suppose.
Combine the two. Yandere RTD gogogo!
Anyway, if you're looking for truly esoteric ideas, I might be able to help. I'll have to think about it some, though.
Color me intrigued, go on.
What, the Yandere RTD?
Players play as bugs, whether fully anthropomorphized bee-people or giant beetles that can talk. Takes place on an alien world, so there are no fortresses made of sneakers or roads to watch out for, but there might still be things the players can't fully understand or explain.
Playing as a bug presents a number of unique challenges. For one, enemies can be truly freakish without being all that unusual. Normally, when one is assaulted by flying enemies that spit acid, then ambushed by a savage predator that can turn invisible at will, then nearly ensnared by a cave-sized plant that secretes glue to trap victims, then wisely realize that the beautiful maiden singing in the distance almost assuredly wants to eat you and all your friends, any reasonable person might start to suspect they've gotten on the bad side of a very inventive wizard. In this case, however, that's just the way the world works.
Secondly, environmental challenges are rather different. Rain could severely hamper flying or even walking, and being on the ground during such an event could easily get you swept away. On the other hand, falling damage isn't likely to be nearly as severe- some characters might even be light or armored enough to be immune to it completely. Flawlessly maneuverable flight or the ability to climb sheer vertical surfaces might be much easier to accomplish as well.
Third, character advancement and creation could be interesting, to say the least. While equipment isn't out of the question, a lot of the focus could be on acquiring food, either for energy (to power or learn special attacks or abilities) or nutrients (to upgrade, temporarily or permanently, various physical or mental aspects).
Finally, none of this would preclude any of the issues normally present in such an RTD. Maybe a small outpost has been losing members, and they need help determining what's causing it. Maybe an ant-like colony is suffering tensions regarding just who's going to succeed the current monarch. Maybe a smuggling or slavery ring doesn't like the way the PCs are snooping around.
Players take the role of various horrors of the night in a quest to bring about their own twisted vision of the future. Various factions and individuals (vampire hunters, King's Guard, sorcerer's cabal) might oppose the PCs, others (obsessed researcher, smuggler's ring, different sorcerer's cabal) might be working with or against anyone depending on what's in it for them, and some (revenge-obsessed madman, demonic cult, third sorcerer's cabal) might wholly approve of and aid/serve the PCs. Setting could be anything, though I'd prefer medievalish myself. Might work especially well with steampunk, if you'd prefer.
The game would have a number of goals. In overarching terms, of course, the PCs all want to enact their visions of paradise- for vampires that might include blotting out the sun, demons might want hellish torture camps everywhere, and tentacles abominations from beyond the stars might just want to summon a piece of sanity-cracking home into the sky. Whatever their specific plans, it should probably be accomplished via the use of fairly standardized rituals, so as not to disadvantage those with more ambitious or impractical plans. Cracking the boundary between two worlds is a good default method, since it allows the change to be gradual but difficult to reverse, as opposed to "Hello you all serve vampires now" or "Some areas are controlled by ravening demons but them and the human regions aren't really fighting."
Needless to say, the closer the PCs get to their goals, the more hellish the world (or at least region) becomes, and the more parties become interested (or outright desperate) to stop them (or maybe get on their good side). There's also the issue of conflicting visions- PCs will have to decide if they're simply going to share the world, or decide who gets to achieve what through different means.
Beyond that, there's the difficulties with operating among those who are clearly not the PCs own kind, and satisfying whatever unusual dietary and perhaps recreational requirements they might have without arousing too much suspicion. A vampire needs to eat, and a demon might have to torture people to death for no apparent reason. Hiding these incidents from anyone who might object to them might be a project in its own right.
Players take the roles of demons (or rough equivalents) and lead/assist in a full-scale demonic invasion of some mortal world. Largely a "There's a war going on, so go run around and be heroes for your side" type of deal, except a lot of the PCs will spit acid or eat souls, and there's a lot more treachery, power-vying, and general incentive to ensure that you're making sure your side wins in a manner that's beneficial to you personally. Some people might be duty-bound enough to warrant it, but generally speaking dying to accomplish your goal, for instance, is absolutely insane- even succeeding without getting a reward from it is a wasted opportunity at absolute best.
There's a hierarchy, so finding someone to take orders from shouldn't be too hard, but as mentioned it's not as simple as "This is my side, I should follow orders from this person." Finding and cultivating patrons who do and provide what you like could be important. Sabotaging rivals is largely expected, but go too far and you'll be more trouble to your superiors than you're worth- some (N)PCs might be able to rise above this by being good little obedient soldiers, but they're also pretty much guaranteed to be sacrificed or shafted any time their superior feels it's worth it.
Despite the supposed war focus, much of the game would be focused on the PCs personal holdings, abilities, enemies, etc. Corrupting powerful enemies to your side, acquiring your foes' wealth, and laying claim to their lands, livestock (or "inhabitants," if you prefer), and other advantages would be far more important than actually removing them as a military obstruction.
And now I'm too worn out to suggest Classy Villain RTD or Power Rangers RTD. Well, the names give you the gist of it anyway.
Man, I stopped bothering to count the number of new replies.
@dermonster: It's always seemed to me that it's the overly simple RTDs that die off kind of quickly. Even the ones that die from overcomplexity seem like they get a decent handful of turns in first; whereas the GM and/or players just sort of loses the will to update the simple ones, despite it being pretty easy.