A couple of thoughts.
Tracking: - If you are not yet tracking your users, start doing that (Google Analytics, Flurry, etc.). The analytics will give you great insights into user behaviour, retention, churn rate, segmentation, etc. Use the data to decide what you should focus on next (e.g. if the user engagement is too low, try focusing on that).
ASO - Optimize keywords to ensure that you are ranking in the top 10 for each of your keywords. I have written a series of blog posts about this topic (http://blackboardmadness.com/blog/app-store-optimization-aso...) and also developed my own keyword selection/scoring model. I'd be happy to help with this (szilard at blackboardmadness.com).
- Optimize file size: got a warning on the Android Play Store that the file size is very large (probably wouldn't prioritize this too high at the moment, though)
Review/ Rating: - Allow users to rate your app from inside the game (e.g. use Appirater to prompt users to rate after X app sessions)
Social Integration: - Deep FB integration (add an invite friends button, post achievements, FB leaderboard integration as in Dots)
Good luck!