I think it's odd how you can vassalize a fanatical purifier nation, and then ten years down the line integrate them into your empire no questions asked. They just go "we accept our integration into a filthy xeno empire." and I'm just like ???
Examples aside, I think it's a bit silly for integration to be mandatory because not every empire would accept becoming merged with another, especially if their ethics differ. They might accept being a vassal, certainly, and probably would happily fight alongside their lord if it meant their own survival, but an egalitarian being integrated into a dictatorship? Those kinds of situations should be when an empire says "We like and respect you, but we refuse integration." It would certainly help out roleplay in that respect. All that said, I feel like integration should be reusable by yourself and other empires, the latter basing the decision solely on similarity in ethics.
Examples aside, I think it's a bit silly for integration to be mandatory because not every empire would accept becoming merged with another, especially if their ethics differ. They might accept being a vassal, certainly, and probably would happily fight alongside their lord if it meant their own survival, but an egalitarian being integrated into a dictatorship? Those kinds of situations should be when an empire says "We like and respect you, but we refuse integration." It would certainly help out roleplay in that respect. All that said, I feel like integration should be reusable by yourself and other empires, the latter basing the decision solely on similarity in ethics.