Extra space for one. That's pretty much the only extra I need at this point. As I add more and more to EN, I am beginning to use up my storage. I don't need any premium features, but would be willing to pay something for extra storage. Just like Google offers with drive.
One other feature I'd considered was a receipts manager. I know an app/extension exists for that but I haven't gotten around to looking at it (just like my taxes, lol). But if it was beyond a basic solution, I'd pay extra.
Evernote's greatest strength - its versatility - is also its greatest challenge, because there are so many users with very different needs. You have individuals using it for things like research, dissertations, small businesses, giant corporations, project management teams, etc. That's why I suggested a la carte, or at least different types of bundles for different populations. I just don't think there's going to be a one-size-fits-all solution that makes everyone happy.