We need to see the whole space and ideally a to scale floor plan of the space showing windows , doorways where those lead and all measurements clearly marked . Posted here in jpeg format in a comment .

Put it on a cabinet (use one you have) in front of the niche to see how it works for a week or two. If you like it, fine. If not, buy a smaller tv and cabinet that will slip inside one of the niches.

The niches look a bit deep so the TV should be positioned at the face of the opening.

I would just put it on a temporary console in front of the niche for now and take my time to think through all the options.

The one thing I would not do is place it over the fireplace.

Are there any other walls? A floor plan and dimensions would help people give you better suggestions.

I'm not sure about how to mount the tv into that rock, but do be careful A friend house had her fireplace chimney damaged because they actually drilled to deep and hit the chimeny. Not good.... But you idea of mounting tv above mantle is the least expensive option. Whichever way you end up, the space will look best if you keep both sides balanced. Here are some future ideas for thought.

Your tv mounted above fireplace:

TV mounted above with freestanding cabinets on sides

Built-in cabinetry and smaller tv to the right of fireplace:

TV mounted above with built-ins and same mantle

TV mounted above but a different mantle mounted lower than current one.

I'd like to update the rock appearance, so I'd recommend you use mortar to fill in the stone gaps and then install a Mortar Schmear over all the rocks

Thank you all for the suggestions!!!!

you could also mount the TV vertically in a niche and then pull it out and rotate it to view it horizontally

what is on the right? Can you use a inverse lift system attached there on the right, with tv hiding behind false wall when not used

PappaB

Ideally you'll mount the TV anywhere except over the fireplace.