It's too bad that most of the time, ROM hacks are either simple graphics replacement or new levels that are ten times more difficult that those of the original game.
I think it's telling that of all the hacks I've tried, only the first two mentioned by Wario Bros were enjoyable experiences. Hacks like Super Metroid Redesign don't do anything for me because they almost require you to already be an expert in the original game and know all the tricks, moves and bugs in order to advance. I just want to have fun and be surprised, not tortured to death!
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Current projects: Bucky O'Hare (NES), Metal Storm (NES), Clock Tower (SNES), Ristar-The Shooting Star (Gen), Sonic The Hedgehog (Gen), Sonic CD (Sega CD), Mega Man Zero (GBA), Battletoads (NES)