So I've heard the synopsis about the movie being a horror film and the group being in a facility, so here is an idea I posted elsewhere:
The facility could easily be a mutant detainment center (an illegal facility; early Transigen) where our young mutants are being held for covert reasons.
The young mutants at the facility have come to rely on each other in surviving the horrors of "The Center" (testing their powers, experiments) by leaning on each other, protecting one another as best they can.
Dani could be the newest mutant brought to the center but with her, she brings something else... Demon Bear. She tries to stay to herself but eventually makes friends (Rahne, Cannonball, Sunspot and Magik). Dr. Reyes (Dawson) is one of the favorites among the mutant captives out of all the doctors at the center because she actually cares for them and tries to protect them as much as she can.
As the Demon Bear's attacks become more violent (which the staff believe is Dani's powers), her friends get involved but the whole facility is eventually pulled into a nightmare and no one is safe. As Demon Bear cuts down young mutants and staff one-by-one, it's up to Dani and her friends to stand their ground and stop the monster once and for all.
I would have Dr. Cecilia Reyes actually be working undercover for Prof. X and that's how they end up at the Xavier Institute in the sequel.