I had a similar idea, and built a prototype for it a while back. Origianlly I intended to use SNES controlers interfaced via the paralell port, but that was messy and mostly a waste of time. Next time I get around to working on it I'm going to switch to using a 4 player playstation->usb adaptor and playstation controlers. The obvious advanatge of this is that I can use things like steering wheels and whatnot without changing any configurations.
I also had Atari 2600 on my list (how could you not!?), that is proving the hardest to do right. Originally I planned on using MESS for 2600 emulation, since it would allow me to arbitrarily map analog joystick axis to paddles and the like, a feature stella lacks for no readily aparent reason. But MESS sucks for 2600 emulation, alot. So I decided to use stella in the end, and have found a way to fool the stupid thing into thinking that my controllers are real atari controlers interfaced through a stelladaptor, so now I can play 2600 paddle games with the analog sticks, and with a few hacks I could have specialized paddle controlers, and even driving controlers if I really wanted to do that much work for the 2 games or so that use them. Stella still has the problem of not wanting to fill the entire screen, but its better then the problems I had with MESS.
Instead of using a frontend (I couldn't find one that worked the way I wanted it to), I just setup ye old explorer to act like one. I plan on building a special "remote" to control it, it will have an exit button, the arrow keys, page up and down, backspace, and enter. All those controls are nessesary to navigate the "frontend", but I also use them for the 2600 pannel controls.
Also, it works great for playing video, CDs, ect.