Beside all the things that were already mentioned by other players (background blocks, more NPC's / Enemys, etc.) please allow me to add one more thing that i would love to see as a level developer. Here it comes:
I would appreciate it to be able to share my work with another player. Not in the way that it gets published so everyone can play it. I mean it would be great if we could both work on the same map with before it is released.
Thats the only thing i have to add. It would be nice to be able to develop a map that was not only build by a single developer.
If there were other level designer that feel like they could need this, please rate it up so the devs of the game can see it. Thanks. ^^