It should be sequel of ... [SPOILERS]
I usually don't like horror movies, but this I watched with pleasure.
To make it perfect I would remove some scenes with most terrifying transformations. There were a bit grotesque. Maybe I would keep some of her "skin problems" to keep the movie a bit dark.
After such changes this could be the sequel of The Man from Earth. Story of another eternal human but with some darker twists. And in the third part, John Oldman and Luise should meet. That would be something ;).