I would consider granite or a hard surface material - in black to bring up the color of the fireplace and set it off. Maybe find one that has a mix of gray. Let it make a statement. It doesn't look too large, a shop would probably have a cutoff that would work.
You could try a really great looking mirror like you see in one of our designs posted below.
I LOVE this "blob" in the granite! I think it adds so much to it. Granite is a natural stone, so this is how nature made it :) I wouldn't hide it all...but that's just me!
As for your towel bar and what to put over the commode, please think about placing art (a framed print) only over your toilet. All other options come with the risk of something falling into the bowl! Not shelves with knick knacks, a towel bar, etc.
It appears that you have a blank wall NEXT to the toilet and that's where I would put a towel bar (for decorative towels). If the shower will be used, consider towel hooks! They are much more practical for everyday use and the towels dry just fine. A wet towel draped over a rod always looks messy IMHO!
If you've completed the bathroom, please post updated photos :)
If I had to put a tv on a fireplace wall, I'd use a tv cover- up option. It's an appealing choice.
But I do have a bias, having grown up in a mid-century home fireplace stone work was lovely and tvs came with no wireless and smart tv options. Antennas were the norm and only 4 channels available. Things have changed so much but I've never lost my love of a stone fireplace wall, with little but the stone and fireplace.,