The first thing that comes to mind is, for me anyway, the long-awaited Proving Grounds concept. The idea of content that solo players can use to hone and demonstrate their skills interests me greatly. But how would it have a lot of new content for players? How is it going to be implemented? And that's even assuming it is Proving Grounds at all.
Since we're speculating, why not speculate more? Here's a few ideas for what the feature could be, all of them pretty much based entirely on what I'd like it to be. One thing seems clear to me, that being that this new feature won't be something like tri-spec, based on the emphasis on new content for players. As cool as it might be to be able to switch between all three of your class specializations, that's not content.