The tools for game development get better and better every year. What used to take a highly talented art team a months to make takes only a few people days to make. Check out this amazing demo reel of the newest version of Z-Brush. In particular, check out 2:50 into the video where they show using a 2d concept sketch to quickly model a 3d object. Absolutely amazing software!
I hope to set aside some time soon to play with it to see if it is realistic to sculpt 20-30 characters for this indie project. My guess is it will be too large an effort per character for this project due to our small team, but I sure hope I am wrong! If it pans out, the first concept alien I want to see modeled? This one: