Ghost sound hard to program and looks kinda OP but really cool, dont get me wrong.
The weeping angel actually is awesome and a fresh concept in general.
The harpy would be a good adition, but sound more like a harder version of Tulpy.
I don't catch too well the disguise mechanich. Do you mean like a pred that can disguise as another NPC? Sounds cool but can't relate to a monster/fantasy creature with that hability that can work on.
If I could choose... I will go for the ghost, I know its hard to programm but like he/she reminds to that little ghost enemy that got scared when you are looking at him from the Super Mario Series, kinda love it. Maybe the ghost can be easiest to escape from his belly because... Well... Things can go trought him.