A good subtitle may turn out to be an important factor from the marketing point of view. I think it should be catchy and suggest what can players expect from the game. The subtitle on KS may be good as a first sentence of the description but it's too long for small pictures on sites like GOG and Steam.
Backers shared a few proposals in comments on Kickstarter. There can't be too many to choose from, so I picked only four. My favurite is one of them and two remind of classic XCOM games. The fifth is KS subtitle.
I don't know if I can add anything later but if you think something else would be a better choice, we could add it or even make another poll.