I wish they'd go back to the old distinction between normal and heroic at level cap. I enjoy properly heroic dungeons (Cataclysm style), but they're not very PUG-friendly. I also enjoyed the easy zerging of WotLK heroics, where you'd steamroll through the place for the fastest possible valor gains. Ideally we could have both, with normal modes that give fast justice/valor and harder heroic modes that award random scenario-style loot.
Proving Ground says hi. All we need is a daily/weekly quest that awards valor depending on how well you do, and solo heroics are a reality. Getting valor for each new wave you reach in Endless would be quite entertaining.