For me, my favorites are usually RTS, RPG (ideally real time RPGs), MMORPG, and to a lesser degree turn based strategy and turn based role playing. I usually like 3D better, but well done 2D is perfectly fine as well.
Well I have this 2D sandbox game idea in mind inspired by Terraria. It would be more technological than Terraria, and even though it's 2D, you will be able to go indoors. I can't explain myself very well, but I'm trying to say like a door on the wall in the background that you can enter, and if you do, its like another layer behind the previous one. and it can go a few layers back. It saves alot of space. So Imagine Terraria with like multiple layers of tiles. Good / Bad / Dumb?
I think it could work. There have been a lot of top down games with multiple levels, but I can't remember playing any side view games with this kind of system. I wonder if it will be harder to make it clear where the doorways into the rooms outside of the screen are? (that is, if the character is facing into the screen with his back visible, the door would be behind him) I guess you'd just put a sign on the opposite wall or something? I can think of a few other issues like how the back walls would work (in this case walls between layers) and whether you'd be able to see into deeper layers for drawing the backgrounds, but I think they could all be worked out.