Assimilating everything into one species solves the problem of an overfilled and unmanagable species tab, but it also removes some roleplay and gameplay features from the game. It would be neat if you could use the assimilation to consolidate unwanted species together, but keep some core species seperate.

For example, this would allow you to have one ruler species and one slave species. Assimilating everything into the slave species keeps your rulers clean and untouched by xeno filth.

As for implementing this, I'm not the most experienced modder, but I think I already got it working in a local prototype. I added a new trait trait_total_assimilation_target that can be used to mark a species. Then modified the total_assimilation_effect to check for any species with the trait exists and use it as the target. When there is no species with the trait, it just defaults to the root species as before.

Pastebin with the modified files:
common/traits/Ktass_trait.txt: https://pastebin.com/7D4PiUF8
common/scripted_effects/72_tass_scripted_effects.txt: https://pastebin.com/3CAJNxaZ

There should probably also be a condition to only allow total assimilatioin citizenship for species without the target trait, but I haven't looked into that yet.

What do you think?