I was just playing and the Iridium kept begging for peace from me. I kept denying because, well, its just easier to take over their homeworld and enjoy the fruits of their labor. That left them a final planet to which I agreed to peace, requesting a ton of their tech in return. The only problem is their last planet will be flipped in 10 turns now.

How about an option to incorporate what is left of an empire to be under your protection for a tax % you agree upon where your cultural influence will no longer flip their remaining planets? This would allow you another trading partner, not having to destroy their empire/species making the galaxy a little less boring. Now if you fail to protect them or something maybe they could rebel, I don't know, just spitballing here. 

Anyhow, hope to see this option in the game as trading and the galaxy will be more interesting with some version of this.