I don't know if this is secretly a feature or something that I don't know of, but I thought it would be cool and add more strategy to the game if the Species you unlock give more than just a physical change to the invaders. I think it would be cool if, at the very least, different species are more likely to get certain traits than others, like the demons being more likely to have the "scary" trait or something like that. Or at most they could have entirely different things they are good at, like the elves do more damage with magic attacks, the catkin are more likely to succeed when being sent out on expeditions, or the slimes have a higher chance to dodge attacks against them.