I actually would like two hacks now that I have completed Red++ 100% with all 205 Pokemon.
The first is a Red/Blue hack with 251 Pokemon including all of of Gen I and Gen II Pokemon, which I know is possible because Red/Blue DX now has 254 Pokemon in total. With a Red/Blue 251 hack, you can still keep the Ghost and Fossils IDs as 252, 253 and 254 too. The story can be identical to the original Red/Blue, or new post-game content can be added to account for the Gen II Pokemon.
This one would be cool as I've ALWAYS wanted to see the complete list of both Gen I and Gen II Pokemon in Red/Blue/Yellow, and it would make trading with Gold/Silver/Cystal more complete if both games had an identical list of Pokemon.
The second hack is a Gold/Silver/Crystal hack that keeps the 251 Pokemon in place, but instead of starting in Johto, you start in Kanto, like in Red/Blue/Yellow, and get to choose between Charmander, Bulbasaur or Squirtle, and then you progress through the Red/Blue/Yellow storyline, aka, Kanto region, win eight badges, the Indigo League. Then you progress onto the Johto region, keeping all of the previous 151 Pokemon and then you get handed either a Chikorita, Cyndaquil or Totodile, get all of the badges and then face the Johto League.
I know this would require work too, like restoring missing Kanto areas, and making sure that Kanto goes first before Johto, but it would be a great hack that would make more sense to the Anime series than the existing Gold/Silver/Crystal games.
EDIT : Thanks to permission from Mateo, who is allowing me to use his Red++ hack as a ROMBase, I am removing all of the new Pokemon he added, aka 152 to 205, and replacing them all with the Gen II Pokemon, aka Chikorita at 152 to Celebi at 251. This would also allow the Ghost and Fossils IDs to remain unchanged. This will basically be called Red 251, and eventually I want to re-enable trading to Gold/Silver/Crystal, but that will take more time, and I take no timeline for release, because as of right now, my deal with Mateo only allows him and me to use it, because it is his ROMhack and he did all of the hard work. Maybe one day, we can allow a public release, as a sidehack to Red++.