Let's just jump right into it. What stuff do I think would be awesome?
An overhaul of the leveling/alt process
I definitely think we need to reconsider how we deal with alts. I'd love to see a system that reduced the time you had to spend getting them leveled once you got the first one to max - heirlooms work for that, but that long-rumored heirloom tab would make it a lot easier. Another thing which would definitely help are more heirlooms like Hellscream's Decapitator - our current heirlooms don't take us all the way to max level the way it will, and it's also useful now as a raid-level item an alt can make use of. It's an experiment I hope they continue.
I've seen some arguments that we should be able to start a high-level alt to skip over a lot of the leveling process, especially as we get closer and closer to the likely new level cap of 100. If that was implemented, it would need to be done carefully, but I'd definitely support some way to preview a class at or near max level so you could figure out if you'll like it or cut down on having to do the full 1 to 100 (or whatever it ends up being) march again.