Add abilities, held items, and non-original 151 Pokemon - or, if that last one is really too much, just add in the cross-gen evolutions/pre-evolutions for gen 1 Mons. Those are the main sticking points for me.
Also, smaller one - improve the following Pokemon graphics. They're not terrible, but it sort of takes me out of the game when my Pokemon are constantly getting stuck behind things and phasing in and out of their Pokeball.
Ooh, one more! Give more NPC Pokemon 4 moves. I used a Ditto, and it's honestly shocking how many have 3 or fewer, even late in the game.