Thanks for your replies! 

You're right, I should have picked one thing and asked about how to improve that single thing and not post all my work at once and expect feedback. 

You've guessed it already. The stuff I want to fix the most are the tiles/tilesets. Right now, I have a pretty flexible setup which lets me assign a certain texture and colors to each single tile. I have a sort of tile "prototype" which contains all the variations in texture:

Then I have several color-sets, each 3 colors. The black, white and read pixels in the texture get replaced by the colors in the chosen set. I can mix texture and colors as I like but in practice, only a handful really make sense and look ok. I might have to break away from this method completely and create traditional tilesets. I just thought I could be lazy but now I think it really shows... 
And yes, I currently don't have any transitions between the different tilesets. You think that's a must?
I kinda fear that 8x8 is too small as a tilesize as I don't even know how to produce tiles which are tileable and still not feel natural and non-repetitive. 
I'm just fishing for ideas right now, so please tell me whatever comes to your mind. 

Hello there!

I am a lurker of this forum, pixel art admirer and game developer. Every once in a while, I see pixel art projects started which are destined to become a game in the end. Unfortunately, many of those projects get abandoned half-way through, even if the project "members" have already produced a big amount of graphical art. This kinda makes me sad.. 

I am on the other side of the medal. I am constantly looking for freely available graphical resources to use in my games. I've already switched to making my choices on which game to develop next solely based on whether there is enough graphics art available. But good art it is still very hard to come by.

Isn't there a possibility to join forces in some form or another? Don't you pixel artists have many creations lying around on your hard disks which you would like to see come to life in form of a game? I would gladly use any existing art and turn it into a game, even if it's just a little short one. I am open to work on (and have experience with) many different genres: RTS, platformer, shooter, ..

I will neither offer nor request any money, I'd like to focus on the fun instead, as I'm doing this in my free time.

I'm mostly an Android developer, but I also develop for PC and Mac. As a demonstration of my abilities as a game developer, please refer to https://play.google.com/store/apps/developer?id=IMakeGames which contains my two recent games, Planet Wars and Jungle Mayhem. (Note how both use freely available art).

Maybe we can create something fun together! Thank you for your time!