Achievements, milestones, exp, leaderboards and badges. Having many different gamemodes, rewards and activities all in one place and surrounded by a single community is what sets Tower apart from all the other games that do aspects of it better. This advantage should be used to its full potential. Also, they'll make everything across the board feel more rewarding and fun and give players something to work towards, while also enticing people to try out different activities. And I just like the notion of collecting colorful Tower-related badges, as I'm just a simple man.
After that, the Arcade. It's one of the really fun-sounding activities that can't be found in any other game. It will help set Tower apart and bring in new players. And I won't lie, I'm personally just a fan of arcades in general and having a virtual one, without the hassle of travel or the need to pay sounds attractive to me.
Finally, the workshop. It'll allow for way more costumization and player-created content, which is great for obvious reasons.