I'd like to write a darker sci-fi PSL idea based around the idea of a person being used as an involuntary alien surrogate, the two characters being the surrogate and a "helper" role. The helper would be the person who breaks them out of the place they're being held. The "helper" would be infected by one of the very aliens the surrogate is hosting, or somehow manipulated by the aliens, so they would start to try and turn on the surrogate to further their surrogacy through psychological manipulation toward complacency.
I would prefer something along the lines of an actual "pregnancy." I'm thinking something darker, not a fluffy line where a boy and girl set up house while she's an alien surrogate. My main desire in writing these lines is to get some angst sci-fi fix by playing something that can't happen.