Before trying to incentivize Castle Ravenloft, perhaps you should ensure it works first. This is probably the biggest reason no one completes it.
In the past two days I've experienced groups with two healers, two dps and a tank several times. (As an aside, this has also happened in REQ with LoMM and others.)
Additionally, the sisters STILL have the aggro bug where the book holder pulls all aggro, and Strahd's orbs do not consistently display before detonation. Can you guys :please: fix these things before forcing this into queue? It's an unfinished and unprofessional piece of work and reflects poorly on your company.