i would like to have a hack for Legend of Zelda vanilla that added 4 things from Zelda - The Legend of Link http://www.romhacking.net/hacks/2136/
1. sword swiping and spin charge attack
2. saving in the inventory menu
3. the select 3 items with the "select" button
4. having a map for sale with Dungeon locations on it
also the automap hack would be amazing to add as well. http://www.romhacking.net/hacks/796/
and also (not sure if these have even ever been done, but) Pegasus Boots, Roc's Feather, and Zora Flippers in a shop somewhere, they wouldnt have to have functionality (be needed for progress) but they could still work.
actually the old man at the beginning could say "It's dangerous to go alone, take this sword and seek out these dungeons" and give you a sword and the dungeon locations map.
maybe the Roc's Feather could replace the stepladder and have the same use, but give an added component in battle
likewise the Flippers could replace the raft, and would allow you to access those areas that way.
but mainly i would like sword swipe/spin charge, select to select between the 3 items, and automap. those alone would make the perfect Zelda 1 re-play