I'm digging what Deflagratio is saying.
Trade should have some additional benefits other than just credits. Infact, all of his ideas are pretty good, but they need some flavor rubbed on them to ensure that it's unique for each of the races, but still balanced... could be difficult though, keeping balance with that - as the TEC already has a pretty big advantage on the trade front.
+1 Karma, bro.
Well, everyone gets trade routes, but I think this would give TEC a little kick in early Games like I think they need. Advent get an amazing IN-culture buff and get their culture research quicky, Vasari get access to wormholes and quick jumps to other star systems and their scouts are also Neutral resource capture units. TEC gets their measly 2-3 credit (before tax) bonus, assuming you haven't used all your Logistics slots yet for research labs. I suppose for more flavor, differences could be:
First off, the Maximum Ship Cap bonus is universal, there'd be no other way to really balance that, heh.
Alliance cap can probably be universal as well.
Advent: in Lieu of Faster Build times, Increased Shield effectiveness (Points and Mitigation): The collective will of the Unity is fortified through trade networks allowing besieged or contested worlds linked in trade to draw upon neighboring planets to bolster their shield effectiveness.
In Lieu of increased mineral Extraction: (Since no real refinery ships) Culture Rate bonus: Each world linked in trade gains a slight increase to Culture planetary value, with enough links, the Will of the unity is so strong, it can begin to more easily overwhelm even the most devout followers of "lesser" beliefs. Works in tandem with the Temple of Communion.
I havn't spent enough time with the Vasari to really guage what they could make most use out of, their Phase Mastery is pretty solid, Probably something to do with Nanomachines or being opressive dicks.
Someone suggested giving Siege frigates kind of an ability to drop soldiers instead, as long as there is no ground battle gameplay that'd be good, instead of completely bombing out the planet, you could take over the entire infrastrastructure, saving you thousands of resources at expense of greater time to take over.
Something I'd like to see also is an "Artifact research treaty" for alliances, allowing allied, or at least people with the treaty in place, to pay (A lot) to send research teams to research any artifacts controlled by another empire with whom you have the treaty. If the owning player loses the planet, everyone who had research spent not only loses the bonus, but also has to research it again (Going back to those important choices Sins is all about). A player (Either side) Breaking the Artifact research treaty also forces the other players to fork out more if a new treaty is reestablished.