After so many years of storytelling it is hard to be completely original. But I get what you mean.
What I would have liked (although also not very original) the movie to be: no modern ghost hunters, just focus on "how did 999 ghosts end up in this house?" That should be interesting enough to make a movie out of it. Some ghosts can be scary, somecan be funny. Think Stardust.
I thought how they did it now was a little sloppy Gracey did a year of seances and all of a sudden all the ghosts from Crump manor came over and settled there? This could be improved, didnt make much sense to me.
And also, so many backstories, first Ben, then the mansion, Gabbie & Travis and we are almost at the end we get Crump's story thrown into the mix (plus some random remarks about Kent, Bruce and Harriet's backstories).
I would focus on the Mansion in the 1800s(?) and tie in a little better Crump's background.
End with the house being overrun by ghosts. Black-out, cut to a title card with 'the present', and while a voice over says "There are 999 ghosts, but there is room for one more," we see a modern person standing in the door opening with the 'sold'-sign. Credits.
If the movie performs well, you can go sequel with ghosthunters. If it doesn't do well, people have a complete story or can make up what happens to the modern person.
