This is a great forum to go wandering for ideas: as is YouTube. While projections can be great for creating the illusion of something coming out of your fireplace, it's not the only one. We use a projection to make one of our tombstones look possessed. It looks as if a skeleton demon is trying to push through the stone. Folks love it.
Atmosfx also makes a video with demons trying to escape a fire that could work very well for your fireplace.
That said, if it were us, we'd probably go another route simply because we already have two video critters at large in our haunt. Take a look a Johnny's Ghosts for wall breakers and other possible ghosts that would fit into your fireplace. He has created a number of animated ghosts that pop out of walls or hang down from ceilings.
View attachment 744203
View attachment 744204
Having one of the ghosts popping out of your fireplace will give it a dimensional look that no video will ever be able duplicate.
As for your fireplace, there are ways you can go from easy to complex. It really all depends on what you're hoping to accomplish, how close your visitors will be to it, and how much time they will spend in front of it.
Simple paintings can be effective if everyone is seeing the prop from a distance. Allen Hopps shows you how to make a crematory oven, using just paint and perspective to give a wall a new look. The same can be done for a fireplace. Add a tiny bit of foam to the dimensions, and you can have a fireplace that from a short distance in the dark, can be more than adequate for the illusion to work.
Or if you really want to go whole hog, you can dive into the deep end and make a fireplace that will fool them all even close up.
This video above is sort of a multi-part video tutorial, because they're creating a whole haunted house, but I tried to set it into the video where they work on the fireplace. The only thing I have to say about the videos is that this is without a doubt the most labor intensive approach to making a fireplace. They could just as easily burned the grout marks into the foam and saved hours. No one would be the wiser. And they could have bought a Harbor Freight foam cutter for 20 bucks
130 Watt Hot Knife rather than the foam factory cutter for two hundred dollars
Industrial 6-Inch Hot Knife #035I. Still, there is so much really great stuff they discuss about painting a fake fireplace and the surrounding house, that it's just fun to watch even if you take an entirely different approach.
Whenever we have a question about something new we want to try, we do two things. We drop by here and plug it into the search engines, and then we do the same with Google images. For a fake fireplace we would look to see which fireplace had the look we wanted, and then follow the links to see if it led to a tutorial. Often they do. It's part of the adventure of Haunting for us. It's that whole "the journey is as much fun as the destination" approach. It's very Zen, only with skeletons and dead things.