Just played ~100 games of this with a friend, tweaking things as we went. Games are very short and random, it's a lot of fun, with something of an addictive quality.. (hence the large number of games).
Showing a new mechanic.. on a whim I made bullets wrap around when they cross the edges, and it introduced this lovely strategic element of building walls of bullets to block your opponent off from a section of the map. Without regeneration, you just don't dare to cross, so you lose access to the powerups over there. Will work on the graphic for it a bit, with glowy trails I think it'll be fantastic.
Had to tweak the rule for taking powerups that your opponents control a bit.. If you can't take them, it's not very interesting - you just get the stuff that's closest to you at the start. If you can just take them by touching them, it's again not very interesting. Now once you take a building it's yours for several seconds, then it starts flashing until someone touches it again - so the opponent can take it, or you can touch it to ensure they don't.
Next: unfair glitchy stuff!