I think at some point the community could try to solve the expansion issue for old console based games (SNES,NES,etc). My understanding there is very little that can be done to add new content without taking away something, and I'm wondering if the community could try to get the best minds together to finally solve the issue.
I just imagine how many dream projects had to be canceled because they found out the hard way that one simply can't expand something like say Link to the Past without sacrificing something else.
That goes more on a game by game basis, and many games do actually have quite a bit of space to work with even if there are some very notable ones that use everything they have. For the most part the solution is obvious -- change to a new mapper, cart type or whatever with more space and deal with the fallout that results from that, possibly including emulator and flash cart incompatibility that results.
Cowboy mode is split the game in two so you have one half with one thing which cannibalised content from the last half and one doing the second part of the game, though this is not always a viable option if most of your space is taken up by something the whole game needs or if slicing levels off gains you not much space in the face of audio or something.
Super cowboy mode, the mere suggestion of which is (justifiably) going to get me dirty looks, is do like the N64 and GC hackers do and have the emulator parachute in extra content. The lua stuff we already see for most platforms being more than capable of this. Bonus of this is you could get really crazy and do voice acting, CGI cutscenes and the like on a NES game if you really wanted.
Has anybody hacked the original GBA version of "The Legend of Zelda Four Swords" to add the DSiWare version's single player mode?
A few people had looked into modding the game to be single player compatible but as it would have taken a whole rework of a lot of the puzzles then it rarely got far.