After minor success hacking away at an snes game I started reading a bunch of threads in psx and n64 rom hacking. Super Mario 64 is a popular rom to hack, although, many projects have been abandoned because of the length of time needed to complete them, and even if they were a good artist they need good programming skills to implement their graphics. If you're a fan of rom hacking check out Super Mario - Last Impact pretty amazing what the hacker was able to do with the rom. He added several new hats and edited the levels a lot.
psX and no$psx are some popular playstation emulator/debuggers. Anyone try to attempt some 32-bit games? I know these things can be a headache and real time consuming, but some that are done right are pretty cool to see how much they could tweak them. Yea with pc games a lot of the modders are able to tweak the games as well, and even more modern ones like Skyrim or GTA5, but there's still a big audience for a lot of the old-school ones. There's like 10 pages for Super Mario World but there's less results for rom hacks past the 90's.