I like the population victory idea. How about allowing vanquished empires to still win this way??!! If the Eee empire is officially in control, but the vast bulk of the population in the galaxy is happy Terrans, then haven't the Terrans really won?
A sneaky player could try to win this way by colonizing in other empires' territory on breathable planets and then gifting them away when necessary to keep the peace. If he did this with all the other empires, and then they used his former colonies to populate even more colonies, eventually his race could become the dominant one in the galaxy -- even without any empire! A jujitsu victory!