How about an achievement system?

Most games have this, it's a great idea to try and achieve goals for certain things. Here's a few examples for this game...:

Reach level 10, 20, 30, 40, or 50 (reward a few gems or whatever).
Reach days survived 100, 500, 1000+ (reward gems or whatever).
Kill so and so monsters, for example; 100 Skeletons, Slime, Mages, etc...).

The list can go on and on. I think this a really good idea to keep players interested for longer periods. To keep them motivated and keep playing.

I hope to get an answer Cowbeans, I'm very dedicated to helping this great game. I really appreciate it. :)