Maybe have an % increase of resource income for having a trade centre built in that system, 1 per system limit and would would either increase global resources per second or system specific. The bonus wouldnt be too big (would require balancing) own a lot of planets, then as you get more planets and more trade centres you economy would grow, once the cap is hit, this would then help allys out as the excess would go to them, or a specific one who needs the extra resorces.
This sort of thing would require super careful balancing, I think it would make it too easy to build up your economy without taking more territory (which I don't want). You might be interested in the Captive Human Settlements, if you have not run across them before. You can capture those from AI planets when you find them, and they give you a +15/s metal/crystal boost, but they have the downside of causing enemy waves against that planet to be 3x as large. If you can defend them, or separate them off from the surrounding planets, then that's quite a nice little bonus, otherwise it's more of a penalty.
Also a deathstar would be nice 
Haha, you should see the "Planet Crusher" ship that I have planned for probably the first expansion. It's not a deathstar, but it's a huge cruiser that is able to destroy player planets (with all the resourecs on them going away, as well). It's pretty wicked. In the meantime, you should check out nukes in the missile silo if you want to obliterate some poor planet. Just watch out for the increase to AI Progress that happens when you use those.
