Here's a review of the hacks I'd personally love to see:
Monster Rancher (PSX) A "Seasonal Shrine" hack similar to this (STARWIN, please come back):
https://www.romhacking.net/hacks/3482/Monster Rancher 2 (PSX) A bugfix regarding how to obtain Mock.
Both of the above A bugfix regarding the occasional loading screen lockout.
Pokemon Red/Blue (GB) Two different patches: one restoring the 39 missing alpha Pokemon like Gorochu, and the other being a graphical patch that replaces the in-game sprites with monochrome versions of the front and back sprites from FireRed and LeafGreen.
Pokemon Yellow (GB) A "Pokemon Brown Version: Special Eevee Edition" hack (with Eevee's voice clips from the Let's Go games on Switch)
Mario Party 1, 2, and 3 (N64) An "NSTC-U Voices" hack that uses voice clips from Mario Party 3, 4, and 5, as well as DK64.
And here's some more hacks I had in mind:
Super Smash Bros. (N64) An "Unlock All Characters" hack that makes Luigi, Ness, Captain Falcon, and Jigglypuff available from the start.
Super Mario RPG: Legend of the Seven Stars (SNES) A hack that adds Luigi as a sixth party member, instead of demoting him to "End Credits Cameo".
Pokemon Emerald (GBA) A "Pokemon Perfect Emerald" hack that not only allows you to get all 386 Pokemon, but also fixes some major bugs, restores the Sailor's original Japanese sprite, etc.
Sonic the Hedgehog 3 & Knuckles (Genesis/MD) A "Kirby in S3&K" Hack that replaces Knuckles with Meta Knight.
Sonic the Hedgehog 2 (Genesis/MD) A "Somari the Adventurer 2" hack that replaces Tails with a Somari-like Luigi.
Pac-Man 2: The New Adventures (SNES) A hack that replaces the Ms. Pac-Man game with the Pac Jr. game found in the Genesis port (complete with music changes).
Pokemon HeartGold (DS) A "Pokemon Throwback 2" hack that makes all 493 Pokemon available, replaces Lyra with Kris, makes the GB Sounds available from the start, and more.
Super Mario Bros. (NES) A "Super Mario Bros. 5: The Uncharted Lands" hack that picks up where The Undiscovered Zones left off.