Thonght I'd throw in my 2 cents (or whatever the phrase is).
Gen IV, more specifically Platinum, was and still is IMO the best game of the series. Storyline, region, mons, the lot. I hope that when the remakes of Diamond and Pearl come around, so too does Plat.
A few small improvements I think could be made to the game;
A true day and night cycle like in Crystal. That was utterly fantastic imo, and despite the attached difficulties in catching certain pokes due to the cycle, I feel it would add a lot of depth to it.
I won't go into a rant on how silly I think mega evolution is, but I hope that the inevitable inclusion of it in a remake doesn't include a similar post game adventure dedicated to it that could otherwise add more true value and depth to a fantastic region and story.
As is commonplace in the series, remakes involve the inclusion of mons from other regions previously not in the game (everywhere you go there's a Zubat I swear). That's fine, but if my knowledge of the world is correct, Kalos and Unova are far away lands. Let's keep it that way. The pokes post gen 4 were almost all awful (I literally skipped XY because of this), if we're going to have any cross region migration, let's keep it limited to gens I-IV.
Mining! As a region without much traversable water, diving is off the menu. Mining however could be expanded upon. Perhaps a more interconnected series of tunnels running throughout the region, perhaps incorporating the Crystal themes of underground rough areas where people battle alongside services and NPCs?
Now, call me old fashioned, but I personally hate the direction gen 6 took the series graphically speaking. Pokemon was never meant to be the 3d animal crossing bs it currently is. I like my top down, somewhat grid like, 4 directional adventure, not quite like the older games (GBC); Unova IMO was perfect, it retained this old feel and had fresh, new graphics with 3d elements whilst remaining true to the traditional style of adventure. Spot on.
It will likely never return to this state, sad, I know, but one can still dream.