I have thought about making a map where the player has the raw deal with the asteroids. The way it is now the centre asteroids are randomly generated, giving the level some re-playability. I wish I had a way to set the AI to defend a specific asteroid. When I was playtesting the AI would often leave its main asteroid undefended to go chasing after my nearly worthless undefended asteroids. Oh well, hopefully we get a patch in a month or two that makes the AI a little smarter.
Really, the main problem is how passive the enemy AI is. Even when it outnumbers you 3 : 1 it will sit there for ten minutes doing nothing. I've looked at just forcing it to send waves of seedlings out towards your own asteroids but I haven't found a way to make that work simply yet, but its the main conceit behind my next map so I'll figure something out.
I'll be refining this more but its really good to get feedback, on my next weekend I'll try posting an insane version and see if anyone can beat it.
Thanks for the suggestions too, I'm working on a couple of other maps and I like the idea of limiting the number of trees on the player side.