It's always good to dream, but i agree with Bygone that it would be too complicated to build those new mechanisms.
A suggestion would be making the changeling something more like a parasite that can leave it's humanoid shell when it's in danger, sounds OP, but with the right amount of nerfs it would be perfect to give the crew a scare and still not make the ling some immortal being wandering around. PS: Not that it wouldn't be complicated to build this new feat also.