I'll be updating this post with each new idea that comes to mind.
Batman: Return of the Joker (Game Boy):
The other Batman games by Sunsoft are really, REALLY good, but this one has one fatal flaw: no extra lives. You die once, the game's over, and you only get 5 continues. Also, most enemies block your Batarangs and your punch's range is non-existent. You'll always trade hits with most enemies if you use it. So here's what I'd like to do or see done to the game:
-Add a lives system with infinite continues. When dying, start immediately at the beginning of the area. Continuing would put you at the beginning of the stage.
-Don't let enemies block Batarangs.
-Batarangs can break crates/rocks.
-Increase the range for the punch so it's more like Batman: The Video Game on NES.
-Remove knockback. (On the fence about this one)
Bugs Bunny's Birthday Blowout (NES):
This game's not bad, but if you're the type who gets motion sickness, the screen scrolling is a problem with how jittery it is. A hack that fixes it to be smoother would work wonders.
Removal of friendly fire, for the love of god. I can't believe there isn't a hack for this already out there. Also, if one player dies the other should be able to continue. Porting over the improvements from the Genesis version would be nice too.
Fester's Quest (NES):
There's already an excellent hack that makes bullets go through walls and rebalances hot dogs so that you need to spend more money, but more work needs to be done still:
-Autofire, so you don't have to use turbo buttons. It shouldn't be as fast as with turbo, as that makes enemies and bosses a bit too easy.
-Increase Fester's movement speed.
-Increase damage done to enemies so they're not total bullet sponges. The PAL version already does this, so it could be ported over.
-Don't remove items that downgrade your weapons, as that's part of the challenge.
-Continuing should not put you at the beginning of the game. Make it so every time you beat a boss, that's a checkpoint.
Ducktales 1&2 (NES):
No continues? No passwords? Come on, Capcom, you're better than this. Adding continues to make it more like Mega Man, as well as an SRAM system, would fix this.
The original game had a save feature, so bring in an SRAM hack that uses the FDS version's screen and music. Alternatively, add an English patch for the FDS version that changes the title screen and save screen to be english.
Gargoyle's Quest II (NES):
The Game Boy version had more levels, and levels taken from the NES version were extended. But the zoomed-in screen and inferior graphics ruin this somewhat. Those extended areas should be added to the NES version so we get the best of both worlds.
Castlevania: Symphony of the Night (PSX)
-NEW VOICE ACTING. Keep the original script, but redub all the dialogue. I'm not even sure how this would work, but it would be groundbreaking in the world of romhacks, something I haven't seen since Ace Attorney Investigations 2.
-Translate "Nocturne" into English.
-Bring in the Saturn version's extra areas, music, enemies, bosses and items. No, I don't care if they clash with the design. More content is more content.
-Alter Richter and Dracula's prologue portraits to match their Rondo designs, and change Richter's sprites to the Saturn version's for the rest of the game.
-Make Maria playable. Whether it should be like the Saturn or PSP versions, I haven't decided yet...
-Implement text changes from the European release and fixes from Japanese v1.2. Change the prologue intro to "Rondo of Blood" instead of "Bloodlines".
-Implement unused items, familiars, and voice clips. Replace the original interviews with the Japanese cast with the newer English cast that would be here.
-Create a new ending based on the unused audio found on the disc. Obviously this one is REALLY stretching it. Also, use "Mournful Serenade" from the PSP version for every ending except the best one, where it plays "I Am The Wind".
-Change the title screen to "Castlevania X: Symphony of the Night" to be more in line with the Japanese version.