I'd like either Vs Seeker or Battle Chateau to make a comeback. Vs Seeker could be improved though ... IIRC, there were times that even if you had it recharded, sometimes trainers wouldn't want to fight you... and that was a pain :C But regardless, I loved using it on Platinum, it was pretty useful! Battle Chateau was great as well ^^ I loved leveling up in there, and it was pretty neat to even rematch Gym Leaders there!
As Hikamaru said, since Hyper Training only works with Lvl100 Pokémon, there needs to be a facility or a feature that helps us train our Pokémon, either rematching trainers or challenging new ones =D