When a team goes up in stage, their structures should change visually(at least for aliens), to show "upgrades" from evolving. Alien structures grow spiky and more menacing, while Human structures get more armored and high-tech looking. Switching the models would not be instantaneous(as this is unrealistic and stupid looking), so the structures would animate as they upgrade(with a 5 second animation, or something). Optionally, structures could be unusable in this transition, and again optionally structures could have increased armor or effectiveness(5hp upgrade for each stage, for example).
I think this would make the game more interactive(and thus more fun).