Stellaris
Did a single-player run that started as an organic empire – something I've rarely done since buying the Synthetic Dawn DLC gave me the option to play as machine empires – and will end as a synthetic one via synthetic ascension. The only thing left for me to do is deal with the endgame crisis; I've already ascended and become synths. Of course, after the crisis is defeated, I intend to keep going a little longer to unlock some achievements and try things out that I never really did before, either because I didn't think to do so, or because I didn't have the DLC to do so at the time.
Synthetic empires are truly an interesting type; they operate like regular organic empires, but the main species are robots. You can't start that way, either; if you want to be a robot empire, you either start as Machine Intelligence and stay that way forever, or you start as an organic empire and pursue the synthetic ascension that turns your empire synthetic... and you stay that way forever; Machine Intelligence empires will consider you a fellow machine empire when this happens, but the way you manage your empire works more like an organic one as your pops are all individuals whereas Machine Intelligence pops are drones linked to an artificial hive mind. You also can't create machine worlds as a synth empire, though presumably you can take them from Machine Intelligence empires who can and be no worse for wear.
I don't know how synthetic empires handle Machine Intelligence pops that enter their borders somehow, but allegedly Machine pops can't survive outside of a Machine Intelligence... There's a Fallen Empire in my current save that is a Machine Intelligence. I could, in theory, build up an overwhelmingly powerful fleet and just kick them over for their planets, just to see what happens to their pops. If they don't purge automatically, I expect them to be assimilated and become synth pops instead. Mechanical pops from other empires survive within Machine Intelligence empires just fine, however, but they lose whatever individuality they had, if any.