Very cool game
Its fine as it is, really, imo... But some things that can make the game lengthier:
#1 Like you said, a "design your tank" feature :p
#2 lots more weapons (possibly a "design your weapon" feature?)
#3 There were a lot of enemies, and I seemed to find something different even when I thought Id seen em all. (this isnt a suggestion, just a compliment :p)
#4 Some type of "random objectives" and a central bubble to go to (this could be where you could upgrade the tank and weapons, etc.)
Great game!