In response to nostalgia:
I've said this before, but most of the prettiest and visually remarkable 2D games used prerendered 3D graphics. Even before that, at the high end of pixel animation, most of the work was being done in an attempt to make the environments LOOK 3-Dimensional. Yes, I agree that there's been recently a lot of focus on prettier graphics rendering at the expense of good artwork (I.e. Crysis), but even back in the day the visually engaging sprite-based games were swamped with visually BORING sprite-based games.
There is absolutely NOTHING to be gained by returning to sprite-based graphicry. That's why the only spritey games that are out nowadays are gimmicks . . . because there's no reason to make a game like that! Oh, and sprite graphics only started getting "good" when they started closely emulating 3D effects. I couldn't even play the early Final Fantasy games when I went back and tried them, because the sheer butchery of perspective gave me an aneurism. They relied on the player to sort out their confusing array of symbols and abbreviations (remember when everything you encountered in a side-scroller was a dot? That dot kills you, that dot gives you health (you can tell 'cause it's a different color) and THAT dot makes you blink for several seconds. It's probably invincibility.
Also, as far as gameplay . . .
A while ago, my roommate got a NES emulator for his Dreamcast, and we spent a while going over the hundreds of old Nintendo games we now had access to. And the thing that struck me was how many side-scrolling Streets of Rage ripoffs there were. It was the thing to do, and there was a version for every conceivable concept, movie, or what-have you.
And it struck me how similar this was to CURRENT three-dimensional action games. Only instead of moving along horizontally pressing one or two buttons to punch, kick, chop, or shoot an endless stream of enemies, you're moving through a three-dimensional field doing the same thing. And it occurs to me that this nostalgia is based off of comparing bad NEW games to good OLD games. Super Mario World is rather more entertaining that Marvel Ultimate Alliance, but that's not because it's in two dimensions: it's because Mario was a quality game, and Ultimate Alliance was crap.
(I was actually going to use God of War for that comparison, but it occurred to me that lots of people LIKE God of War. God knows why.)
So, yeah. Screw nostalgia.
Oh, and one more thing. If you want to focus on gameplay instead of graphical gimmicks? Use the most up-to-date graphics available. Because they make things much EASIER to build. You just tell the engine where stuff is, and what shape it's in, and it'll shadow everything and reflect light just as it would in the real world. One of the biggest problems, for instance, in the old Thief games was that there was no proper system for what they were trying to do, and they had to build the entire lighting system from scratch. Nowadays? Not a problem.