I play Pokemon games primarily as singleplayer RPGs and since day one the singleplayer portion has always, always, always fallen by the wayside. With the exception of G/S/C, every Pokemon game is basically the same sequence of events with more and more and more stupid distraction bullshit piled on top of it.
And that sucks for guys like me. I'm sure multiplayer Pokemon obsessives can provide a list ten miles long of everything different since R/G/B/Y, but seriously - if multiplayer is all that matters, maybe Nintendo should just make a multiplayer-exclusive Pokemon game. Cut out the singleplayer entirely if I'm not supposed to enjoy Pokemon games like that.
I also wouldn't mind a new battle system. Me and my friends back in the day would daydream about a Pokemon battle system that looks like a fight from
Grandia, where you have Pokemon running around an arena in real-time. It doesn't have to be any different
mechanically, but something a little more interesting to look at would be nice.
No more new Pokemon. Especially no more new Legendary Pokemon. They aren't special anymore because there's like 15+ different Legendary Pokemon by now.