Please post photos of all 4 entire walls for context.

If the large blank wall is the area above the TV, my opinion is that it doesn’t need to be filled especially as the filler will be above eye level height.

I think the issue is that the console, TV and etagere/bookshelf look undersized adjacent to the tall fireplace and mirror. Replace with a longer console and a larger wall-mounted TV. Rehome the etagere/bookshelf.

You have already decorated too high on the wall with the mirror over the FP/ Art or mirror over a FP should generally be the size of the firebox.

Waiting for additional pictures.