1. Scaling of all old raids/dungeons similar to challenge modes. Bosses will still drop the same loot, therefore the rewards will be cosmetic only. Transmog gear, titles, and mounts.The unannounced feature will be in the last patch of the expansion and give players a lot of new content to play with.
2. Random Raid Finder. This may not be the feature they're talking about, but will probably be introduced in 5.3. Keep the older MoP raids alive, while giving players extra valor for completing a random raid.
3. Some kind of user created content. Highly unlikely, but had to put it here just in case. I hate to say player housing, because I don't think that fits w/ wow and devs have stated many times they don't like the idea, but something similar is probably to be expected. I honestly thing they are slowly testing the waters for this with sunsong ranch.
Just my 2 cents. Your thoughts?