Some hack suggestions:
Nightmare on Elm Street (NES)
There is plenty of hacks of Friday the 13th, but i could not found a single one of Nightmare on Elm Street.
Some sprite editing to make the enemies a bit more threatening than halloween style ghosts would be nice, and maybe make Freddy a bit harder if possible.
Goonies 2 (NES)
The script is a little stilted and some hints are very vague, so a rewriting of the text to be a bit more natural would be nice.
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.
Godzilla NES -
if someone could get rid of that stupid time limit, this game would be much better.
Castlevania 3 NES -
A sprite edit of Trevor to make him look a bit more like his PS2 Appearance
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.
partly Fixed - pacnsacdave made a hack http://www.romhacking.net/hacks/2638/
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.
Also fixing the horrible hit detection in Wizards & Warrirors 2 would improve that game a lot.
Battle Toads NES -
Just a simple shades and Gloves hack to Different between Zits and Rash.
Toki NES -
Change the sprite of Toki to look more like the Arcade version
Star Tropics 2: Zoda's Revenge
I would like to see a graphical overhaul of Star Tropics 2: Zoda's Revenge.
When the precursor game looks better, you have done something wrong.
Top Gear SNES
I was watching Top Gear, and thought that now when the original trio is no longer on the show, how about a hack of the Top Gear game?
Some things that can be changed is:
Exchange the Title Music to a arranged version of Jessica.
Exhange the racer character to the Stig and one of his cousins as player 2
Change the speech bubbles to quotes from the show like "Oh Cock" "How hard could it be?" and "Pooooweeeer!"
The Legend of Zelda 2: The Adventure of Link
After i've been trying Killerbob's awesome Loz Relocalized hack, i've would really like a relocalization of Legend of Zelda 2 as well, making the script a bit less stilted and more natural, and once for all solve the mystery of Bug & Error which got lost in the original localization.
Final Fantasy II NES
Final Fantasy II NES - Give Firion an 8-bit version of his Final Fantasy Origins sprite, and also make his portrait match that sprite.
Ghosts N Goblins NES
Since i'm a bit alergic to engrish, i would love to have a hack for Ghosts N' Goblins for NES to rewrite the bad engrish in the game.
Partly Fixed - Ghosts 'n Goblins Proofread by Mister Xiado. take care of the Engrish
If it also could be combined with WolfTeam22 Makai-Mura AC - (Arcade Conversion) hack it would be super. By the way implement atleast some continues to this version, the NES version was brutal but atleast you could practice, The Arcade conversion is so brutal you wont see it coming.
Thats all of my hack ideas.