Gah, there would be so many things!
Personally, I have a lot of original games I would like to do. My imagination can run wild and I think I could actually come up with a lot of original concepts. But I need people who can get the work done.
That said, I would love a Populous 2 remake. Populous brought the god-game into my world, and Populous 2 is still one of the best examples I can think of. I would increase the level of detail in every aspect of it, but the basic game would be nothing more than that. The people would still live independently (bar the 'heroes'), and you could still only influence the surroundings. The game would be all about online multiplayer gaming though, this time, with just a fleshed out training mode offline. I would have a beatiful world with lots of great physics (flowing water, sand, etc.) and then expand all the natural disaster stuff to take these physics into account.
Perhaps I would eventually want to try to expand it by increasing the level editor aspect a la the Game 3.0 concepts of Little Big Planet and Will Wrights' game combined. I'd certainly design the remake around these principles, even if they are then used only internally, just in case, and because it makes things easier.
Oh, my. Just imagining this.