[Sei]

I wish Pokemon had a 'new game +' feature, where you could keep the EV-trained pokemon you had in your party (it would save in a second file so you wouldn't wipe out your original file), but they start at level one (with your starter at level five), and all the trainers and gyms were harder and required more strategy.

That would be amazing and I would play it over and over again, because although it was easier to switch out pokemon if I wanted a change, some of the pokemon can only be found at the end of the game and there's no purpose for them unless you breed, collect for pokedex completion, or battle competitively.