How about a completionist hack of Super Mario World? In most subsequent Mario games (including Mario DX for Gameboy Color) the game will remember whether you've gotten all of the Yoshi Coins / Star Coins in each level. I was hoping they'd do this in the GBA release of Mario World, but no such luck.
Here's all ya gotta do:
- If a player collects all the Yoshi coins in a stage, display an indicator while they are standing on that stage.
- If a player collects all the Yoshi coins in the entire game, pallete-swap the title screen or something, like in Yoshi's Island.
I'm sure a Dragon Coins indicator hack has been done in SMW SNES before, as I've watched people play ROM hacks with it.