I'd like the VS. Seeker to return. I found it brilliant, and quite "realistic" too. Your rechargeable thingamajig sends out a signal, asking "Is there anybody in this area who wants to battle?". Then it logs the responses, and you walk up to the trainers and have your promised match.
Imo, it's a bajillion times better than those "trainers' eyes", where you had to move all across the map to find a guy who wanted to rebattle you, and then wait for anywhere between 30 seconds and three weeks before he wanted another rematch. It was totally random who would battle you and when. The VS Seeker worked for all trainers, almost anywhere you wanted, the Trainers' eyes only for a select few.
Well, at least better than the PokéGear, imo. The VS Seeker was good for grinding. The Trainers' eyes are... well, they give you an incentive to travel across the region multiple times, and since that region is Hoenn, I can only praise that.
Also, I'd like rebattling trainers and gym leaders to return. All of Unova's postgame facilities are crammed up in Nimbasa City, including most of the rebattleable trainers. There are random times of day when you can't enter that battlefield, and I dunno, I find that Gyms and other dedicated battle arenas have fields more designed to take a beating, easy to maintain and fix after a rough battle. They could of had a "Pokémon Stadium" or something where trainers hung out, instead of doing all the battling in the arenas where humans are supposed to play. Anywhere but the place where it's kinda important that the field is in good condition before every match? I'd have loved the concept a little more if it was in an area designed (and mainly used) for Pokémon battles. Kinda like Battle Square in Pyrite Town. Heck, there's a perfectly good park with a field drawn up in Flocessy Town. Why not have rebattleable trainers there?