I'm not entirely sure where this should go, so I put it here, since I guess it's an idea that could be used in the expansion.
In the game I'm playing currently, I had a fleet resting on my frontier, defending an enclave I have of planets recently captured. A few months ago, I gave a few of these planets to my Thalan allies, as they hadn't done terribly well in the race for colonies.
Anyways, this fleet is sitting there, and I notice a group of Altarian transports rushing towards a Thalan world. I try to intercept them, but my range falls short. However, I could've reached the planet and defended it -- if allies were allowed.
So here's my idea. Why not allow allied civilizations to defend one another's planets? Expanding on this, why create an ability to "allow" allies to lease ships to each other -- sort of handing them over temporarily, and perhaps picking up some cash by doing so.