Getting back on topic, there is an issue which I am still unsure about: the handling of undesirable traits.
In the low end, there are traits like Repugnant and Quarrelsome, which seem like things that should give a species a harder time advancing in society. There are also traits like Necrophage, which seems like a trait that would be perfectly reasonable for other pops to be afraid of. In the high end, there are traits like Noxious which are objectively undesirable. It seems reasonable that players/empires should be able to react to them somehow, but I am not entirely sure how.
One option could be that some traits enable descent into the "None" Citizenship Type, where they could become optionally purgable, or an even lower "Undesirables" Citizenship Type where all affected pops are subjected to purging.
Another option could be that more traits harm job competition, making those species end up as Civilians more often and thereby be more likely to emigrate.
What other options are there?