Main issue I see with this is you’re going to kill Kasimir’s storyline.

You would also need to explain how Patrina’s soul escaped Barovia in order to be reincarnated since there are no female elves for her to be born as in Barovia.

I'm toying with the idea that my PC elf is the reincarnation of Patrina. Do you think it's a good idea ? Do you see Stradh acting differently towards her?

I was thinking maybe when he saw her at the funeral he could say something under his breath " Alright vampyr you're testing me big time now"

One of my PC as the observant feat so he can read lips. 

It could be a way to introduce the idea of something greater than Stradh

Patrina Velikovna reincarnation