Here's something SPECIFIC I'd like to see: A mounted fighter build or paragon path.
Right now, mounted combat is just "X gets to make your move for you, sacrifice a standard action to let X attack, and you get fiddly plus Y for being on X". I'd prefer something a little more thematically sexy. A cavalier/mongol-style build, with mount-specific powers, would be refreshing. Trampling, a two-attack power that lets you use a basic attack and your mount to use an at-will ability, that's what I'm talking about, a Marking mechanic that lets your mount attack the foe if they shift, etc.
I however see the futility of the gesture, given how often paladins got to take their mounts into the dungeon, unless they were halflings or gnomes riding pooches. And now you have to keep your mount alive just to benefit from your powers.
Granted, I'd never get the opportunity to play it, but just seeing it in print would make me satisfied on that "This needs to exist" level.
Edit: You know, as I think about it more, a mounted class might make a very nice striker too. Movement powers, especially 'mount moves speed, no OA, attack anyone in line'.