Assuming that this discussion is on gc, rather than uc.
As a player who has (and is currently) playing all three free races, I think the best thing that could be done is to improve the way battles work. To me, this would push fighting to the front and centre of the game.
I don`t have any concrete proposals right now but here`s what I think needs to be done:
1. Reward stacking;
2. Make spamming ships difficult.
As it is, its sometimes better to stay in DP because even if you bother to have craft a good stack you might end up on the receiving end of a pyth spam, or a herc/nirv/tyr spam, or a sloop/brigantine spam.
I actually don`t mind the possibility that I can lose good stuff if there`s a decent chance that I could hold on to it if I took the trouble.
Assuming some improvement to the battle system, I suppose one thing that might encourage fighting and good stacking is to reward defensive battles. One possibility would be to reward defenders with an artifact that allows selective clustering (i.e. you can actually tick which colonies you want clustered). Naturally, virals and collectives who have a form of selective clustering (thru selective assimilation/infection) would benefit a lot less but these races are supposed to be more difficult to play anyway. Selective clustering would only benefit those who actually did want to form clusters, and would be preferred over something that allows you to shift colonies without clustering. This "reward" should be made random (to avoid encouraging arranged battles) but frequent enough that those who regularly fight on defense will get some, and the rate could be marginally higher to reward defensive wins.