I think that the defense and Diplomacy trees have too few branches. One is not enough. I think that Defense should get Surface Defenses that add health to the planet, add surface to space weapons, bomb shelters, and those kinds of things.

The Diplomacy tree should have the Reputation branch, which deals with how other empires react to you, and how easy it would be to colonize other planets. Low reputation would mean that empires act more aggressively towards you, and it would be harder to gain good diplomatic relations towards that empire. Colonizing planets would be much easier with high reputation. The higher the reputation would be, the better the buffs would be that the planet got upon colonization. For example, At fairly low reputation, you might gain a 10% build increase; the citizens are fairly happy and help your empire build that planet. At very high reputation, you may gain a permanent buff that increases ship build time by 200%, or adds an additional orbital slot. Low reputation may gain you -20% trade income, or 40% increased cost to build orbital structures. The research tree would have ways of researching new "buffs" for colonized planets, reducing "debuffs" and minimizing the reputation loss.

Also, i think culture should influence reputation, planets affected by your empire's culture would gain additional buffs, depending on how high the reputation was. This way, Broadcasting centres would have uses beyond passively de-colonizing hostile planets and gaining that 1 antimatter per sec/planet view/etc.

What do you guys think of this idea? Feel free to contribute.