Hm seems like I've overlooked this one. As it is obviously filled with placeholders right now, I won't talk about the visual aspect, Error014 gave many helpful advices already.
You said that it is already playable, so go ahead and post a gameplay video! It's pretty hard to judge the latter from screenshots. I'd say drop the idea of a multiplayer component for now completely and focus on your singleplayer game. If it's done, you can still check the possibilty for a multiplayer implementation.
And btw, you may want to look for an artist so you can concentrate on the gameplay, a nice video will definitely help to find one.