This is probably pretty controversial, but trainer customization. It just felt like a cheap gimmick, and I honestly have never understand why people went crazy about it. I just don't think they should waste development time / energy on gimmicks that add nothing to the enjoyment of the game.
Honestly, is there anyone out there saying "ORAS would've amazing, if only I could've changed Brendan's hat. Then it would've been perfect."?
Agreed to a certain extent. I think the basic customisation (i.e. picking skin colour and so on) is cool but I found the idea of buying clothes and stuff quite whatever. It's not an offensive feature obviously but yeah, it'd be nice if they focused on stuff that felt more impactful.