Decorate where the people are! If you nicely decorate on the people level with art, rugs, lamps, pillows, comfy chairs, etc. the rest of the space will look airy and light not empty.

I wouldn't put any art there. The fireplace is the focal point and your eye will go there no matter what. Eventually I suggest you rearrange since the TV looks kind of awkward, but it looks like you have kids so I understand it might not be possible yet.

I agree with no artwork on either side of the fireplace. I have a vaulted fireplace myself and no art on either side. Right now the walls on both side are painted an accent color but when we repaint this spring I'm going to paint them the same color as the rest of the room. However, an accent color may give you the pop you're looking for.

I'd also consider putting a tall plant in the corner next to the couch, unless kids or pets don't allow it.

Replacing the two candle pieces with something in a color other than black or grey and a bit taller than what you have there now would add some visual interest to the mantle. How about adding some seasonal berry strands to the wreath? It's very static.

Perhaps just focusing on decorating the mantle itself would be a wonderful idea. Here is an idea from a design we did for a client:

Alpine Ski Home · More Info

Alpine Ski Home · More Info

Here is what DD did with a similar set up and I think it works well. The shelves are new and not styled yet.

Agree that tv placement looks awkward -- it might be a good idea to mount your tv to the side of the fireplace with maybe a small chest below to anchor it.