i'm not sure what you mean. you can create species (via traits) now
However what Stellaris does not have is any memorable empires. Empire attitudes, preferences, and such, are all table driven which pretty much means they are all the same. Seen one Devouring Swarm seen them all.
What would be interesting is an empire designation that grants the use of a unique set of tables to govern decision. There is an country_type value that could be used to hold named empire types which could have unique starting values along with set build preferences, diplomatic interactions, and more.
In effect you could have a Devouring Swarm Empire which doesn't attack neutrals who favors ships that don't use shields and only mounts missile weapons and researches those by default. I am not sure if research weights are personality based but ratios of hull, armor, and shields, are so its only a matter of extending tables.
Think of it from the space monster side... you see a specific space monster you know how to deal with it and currently you see a devouring swarm by default they must act the same each time but what if we could change that?