I have been thinking about a system to add some content for the casual players, that wouldn't require any new art. (art being the development bottleneck)
By combining the boss scaling technologies from Flex raiding, the item level scaling of heirlooms, and the appearance changing technology of transmog, Blizz could *theoretically* easily revamp every dungeon in the game up to current level.
This would do a number of things including adding an ever increasing list of dungeons while leveling (Ding 68! Time to run Utgarde Keep until The Nexus is available =D).
Also, by using the scaling technology from heirlooms, you could present item drops that are always relevant to your level. This offers a strict sense of increasing power between levels, even at max level.
By incorporating the transmog technology you could change the appearance of items based on a level range. using existing models, Blizz could create item sets that represent an escalation in power as you level and get new gear.
This plays a role at max level by again utilizing the boss scaling technology. By banding mechanics to different level ranges and difficulties, every few levels, and again at max level during heroics, the dungeons would feel and play differently.
This system more aligns with the Battlegrounds that are already featured in PvP, where as new expansions come out, the older battlegrounds still retain some value, and there is an ever growing list of them available. (albeit a little list).
Anyway, just some thoughts I had. Any feedback is welcome. thank you!