Skip the mantel entirely. : ) You don't see it anywhere, because nobody does it. The mantle is the accent, or part of a surround. You will have all the edges, they will need a pencil trim or a bullnose. It is over think. Pick a tile, and if you need an accent tile ? You really don't or you highlight the fire box. Here's why you DON'T find it........: )

As a temporary thing, I’d just have one thing, solid, centered. Because, there’s this triangle upper shape made by the upper woodwork; the soffit extends past the mantle anyway. Small multiple things just look messy. I like the shape of the metal object because it does have that shape. I’d prefer it darker to pick up the stone more. An almost actual triangle or fat obelisk. But I’d not want you to go out and buy just stuff, or obsess about it. To start, just remove the end objects. Another thing you could try, is a single small art painting, just big enough to be between mantel and soffit and a bit wider, some kind of high- contrast abstract, also centered. You could experiment with using just a piece of paper or poster board.

I would just remove the mantle -- at least for a while. If what's holding it up means there are ugly holes in the brick I'd have to decide how I wanted to deal with covering them up -- but it might not be another mantle. If taking down the mantle is too much of a risk, I would remove the artwork, the candles (including the ones on the hearth) and I would set the live plant on the floor, not the hearth. Then I'd think about what else I wanted -- if anything. Probably fireplace tools where plant now is. (or is it gas/electric?) Maybe (maybe) one or 3 framed simple drawings propped up on the right side of the mantle? I actually think the terracotta low-keyness of your fireplace is nice. I wouldn't draw more attention to it but just enjoy the earthy hues.

52" to 56" to the top of the shelf is typical, but it depends more on the height of the fireplace opening and the specifications laid out in the installation manual for the fireplace. If the mantel is wood, it may need to be anywhere from 12" to 18" above the opening to the bottom of the mantel depending on how deep it is. A non-combustible steel mantel can be 12" above the opening. Again, refer to the owner's manual of the fireplace.