I only have two ideas, and they're basically the only two things I'd ever ask of the gaming industry. For the most part, they do their jobs a lot better than I ever could, and have lots better ideas.
I. In a nutshell, it's Smash Bros. in a fully destructible 3D environment, with comic book characters. Think "Hulk: Ultimate Destruction" or any of the free-roam Spider-Man games, but with dozens of playable characters and city-sized environments to do battle in. I'd format the controls much like Smash. Bros, or THQ's wrestling games, where the basic control scheme is the same for each character, but the moves themselves are varied enough to give each character their own unique flavor.
I'd also probably break them into a few rough "tiers" of power, so that you could do justice to, say, Silver Surfer's power set without having to nuke him down to something like Captain America's level. Within each tier there would be a normalization of power balance, though, so that equal battles could be created.
There would be a geeky cameo-filled single player plot that incorporated many of the characters to get used to playing as them, but the meat would be in multiplayer.
II. This is an old idea of mine, and many of its elements have since been created. Free-roam RPG along the lines of Elder Scrolls. Except what I've always disliked in Elder Scrolls is that you could being the head of the Mage's guild AND the Warrior's guild, but actually be playing a sneaky thief-type. From a "lots to do" perspective, it's great. From a narrative one, not so much.
So, it's a Celtic setting, much less magic than most fantasy. And it's entirely free-roam, but there are several "World Paths" (my own term) that take you on completely different courses. So you could spend the game being a thief who takes over the underworld of a shady city or simply loots from the rich to live in luxury. Or you could become lord of an entire area of the country, leading armies into battle and controlling the social and economic stability of your empire. Or several equally as diverse paths between those two extremes. Some might overlap, but you could not do all of them. Classic RP elements and skill customization would be present, and there would be medieval-fantasy style upgrades that you could hunt down and acquire that would cater your character to different play styles (think Deus Ex but in fantasy terms). There would also be some basic turn-based strategy elements that have to do with kingdom management (or organization management for other Paths), but as much as possible I'd want them dealt with in an intuitive way that doesn't take the player out of the experience.
Questing would deal much more with politics and human interaction, and not so much "clear the dungeon, get loot." Assassinations, protecting villagers from raiders, that sort of thing. The world would need to feel alive, with quest trees branching out from actions in such tasks.
The level of detail I've built into this world would require a much larger and more intricate game than any that exists. It would need a functioning economy, cities as big as many games, and quest trees that allow for each World Path to be satisfying and epic (since there's no "main" quest).
...
The first of those is completely possible, and would kick all kinds of ass if done properly.
The latter probably isn't, because the production time would be ghastly, and the elements too diverse to put into a polished product. It's part of the reason I latched on to the Game of Thrones show recently. I was like "well damn, this is basically the book/TV form of my game, just with different settings and characters."