In discussing the idea of invading in a separate thread, I began to wonder... can you set up new pages in the HUD? What I'm thinking of specifically is instead of making the Emergency Facilities infrastructure an individual upgrade, it would take you to another page of planet upgrades all related to defense. Perhaps...
Surface to Orbit Weapons (let's planets attack)
Infrastructure Redundancy (increases planet health)
Underground Tunneling (reduce population loss from bombing)
Bombardment Shields (reduces bombing damage)
Just things of that nature. I'm sure I could find ways to do any of those things, but the point is, can you make a new build tree like that, and if so, how?