I'm a really big fan of Pallette,sprite or text hacks, small touches that improve the game. Here is some of my wishes for hacks, which will probably never be done, but one can hope right?
Ducktales 1 & 2 NES -
Give Scrooge Mcduck and the beagle boys their cartoon color scheme.
Ducktales 2 NES -
Fix the diaglogue so Gyro and launchpad call Scrooge mr . Mcduck or Mr- Mc-D like the original.
Bionic Commando NES -
The no censorship hack is awesome, so now the only thing bugging me is that the Nazi's are wearing neon pink uniforms, a fix to give them grey, black or brown uniforms would improve.
Robocop NES -
a simple palette hack to make Murphy grey instead of blue.
Friday the 13th NES -
this game could really use a full makeover, but till one is made, i'm simply satisfied with a change in Jasons color scheme from purple to blue, and the mask to white.
Godzilla NES -
if someone could get rid of that stupid time limit, this game would be much better.
Castlevania 3 NES -
It would be really cool, if one could remake the sprites on Trevor and Alucard to look a bit more like their later apperances.
The Adventures of Bayou Billy NES -
I really like the mad city version with it's much easier difficulty, but i miss the sound effects and the graphics from the american version, so a simple difficult hack to the american version would be superb.
Back to the future NES -
Well it will still not be a better game, but the music in game is the power of love sped up to unrecognisable. A slow down of the music, and a simple sprite hack to make the ingame person look more like Marty, and it would atleast look a bit more like Back to the future.
Journey to Silius NES -
I heard this game was supposed to be a Terminator game, so a graphic and text hack to make the game look more like Terminator would be awesome.
Double Dragon 1-3 NES
A simple change of Billy and Jimmy's hair just to make them more accurate.
Super Mario World - SNES
With all of the Super Mario World hacks, i am surprised to see that no one has atempted to restore the Koopalings to their super mario bros 3 colors.
Wizards and Warriors 1-3 NES
I have always wondered why the in game sprite of Kuros look nothing like the artwork, don't know if it can be done, but a sprite hack of Kuros to make him look more conanesque like the artwork would be cool.
Battle Toads NES -
Just a simple shades and Gloves hack to Different between Zits and Rash.
Toki NES -
The sprite of Toki in the Nes version looks way to cute compared to the other versions, a sprite make over would be good.
This is what i can think of right now, maybe i will write down some more if i can think of a few more.