You can go back in time and remove one feature from that was introduced in Gen 2, and it would never be added in future Generations.

I'd remove Genders from Pokemon. It's only been a source of unfun mechanics [Attract], unfun optimization for competitive [Breeding/IV/Nature farming], ruining the magic of shiny pokemon [just breed a million eggs till you get one] and unfun source of shitposting, and useless retroactive first stage mons[baby pokemon]. I'm also picking Gender because it'll take out Breeding too since without genders you wouldn't even have breeding as an option