Can you have an interstellar empire?
Much depends on the technology. One panelist observed that on Earth, to have an empire, you have to be able to field troops to a location inside it within three months. Any longer than that, and it won't work.
We discussed the technology involved: FTL travel, and how fast, FTL communications, and how fast. This would have enormous impact on what is feasible. We talked a bit about the optical telegraph in the Napoleonic times and the electronic telegraph. Another panelist knew that the Romans had used a semaphore system. (I brought up longevity, which would change some of the time spans involved, but we didn't go much on that.)
If the communication is slow enough we would need a top-down control for an alliance. An alliance of equals could not negiotiate quickly enough. And democracy would be impossible: correlating all the data and bring it up? Much more likely an emperor with strong viceroys. Also, what about all the information involved? Some AIs to collate it all would be needed. It is, after all, several plants.
Another technology that might be a factor is psychology. The question of obedience might shift if the emperor can test for it, accurately, and condition for it.
What possible motives for an empire? Economics are unlikely, there's very little to be traded once you have an FTL drive. After all, any solar system has iron. Ideology, alliance. . .
On the topic of aliens, we discussed how hydrogen breathers and oxygen breathers could have intermingling empires because they are not after the same resources. Asteroid mining, we might be rivals at. Also, I pointed out, solar energy is limited. A Dyson sphere stage would certainly trigger rivalries. One panelist thought that meant post-scarcity, and I observed that there will never be a post-scarcity society because if it were possible, we are living in it. Another panelist talked about an Intuit tribe that starved to death a few years after a documentary on them; that's a scarcity society. (How we go off on tangents!