I love this game! It is very interesting and has pretty good mechanics.
Are you going to add some maps for surviving as long as possible? Also adding an editor would help to make this game awesome!
That is a very good idea. Last two levels are just what I wanted on this game, but didn't manage to use all possible resources at 100%, so I was a bit upset on that. Adding a possibility to move forward even more will help much :) .
But please consider CPU usage. Unity engine is quite CPU-intensive, adding thousands of enemies spawned at once can kill even best processors. Or maybe you're using some nice optimization?