UnderWorld has a basic 2D pixel art style, which is not bad, but not the best. My next idea is : 

1. Remake the entire tilemap, adding more detail to the ground.

2. More details on the ground. As you saw, the game has a lot of trees, a LOT. So i think maybe adding some new buildings, details, interactive items around and between the trees, would transform the entire game.

3. Enemy Types. Currently there are a few enemy types, including the shelly, which is almost imune to arrows, and more. By adding more enemies and more enemy types, the game's difficulty would rise. Also it would transform the game's aspect, adding color and variety to the map.