Need a photo, please! :-)

How can you take a photo if it's invisible?

LOL Yes I will. We are still unpacking. The walls are Navajo white. We would love brick around the opening, but not sure if this can be added. Also thinking shiplap to help cover the opening the previous owners used for their tv.

Maybe some decorative tile like these? Syzygy

You could clad the whole thing, including the sides with stacked stone or split face and get a chunkier mantel. Easy to drywall and stone over the inset if you don't like it.

Easy change is to try some paint. Pick an accent colour determine how high and wide you want to go with it. Use painters tape to get some visuals. You have lovely high wood detailed ceilings. That way you save on the expense of tile until you figure out colour, style and type of finish.
However you could pick something in a really textured natural stacked split rock, from floor to ceiling. Something very neutral, cause if you go big colour all the way up it could end up looking like a huge exclamation mark!!!