I'd love Dungeon Keeper 3. Never going to happen though.
In terms of original games (which is what I thought this thread was about from the description), I'd love to see an RPG/RTS, where you take a character and go through a game that changes depending on whether you solo it (Which would combine RPG elements with stealth), Pick up a few characters along the way (WRPG-styled), or full-on just recruit everyone you possibly can to end up with a frickin' army (obviously by the end you'd be recruiting entire bands of mercenaries and countries' armies) for a total RTS route thru it. If you took the more small-scale, RPG-ish route, the focus'd be on taking out one main Big Bad, perhaps some monster or villain taking on or taking over the local area and wreaking havoc. The RTS route would involve more pitched battles, with the focus on uprooting and destroying a larger, more amorphous evil, on a much larger scale.
It'd probably be way too ambitious for most developers. So, obviously never gonna happen. Still, that'd be my dream game. I accept that some people wouldn't even like it, too, so yeah. In fact, I HOPE it never gets made. It'd blatantly be bastardised and misunderstood, becoming the next frickin' SPORE.