no mirror, keep tiles same, nice metal

piece bronze, gold nice complement to

Are you placing the TV on the shelf? That's what these deep mantel shelves were designed for.

Paint the tile. Leave the black FP box as is.

How about a mid-size plant? Or sculpture? I'd do something to take advantage of the angle.

I would get a small corner cabinet with 1 or 2 doors and with shelves. Put it up there and leave the doors open, and fill the cabinet with interesting things, including some books. This cabinet is probably too big, but if you found a smaller one and just took the feet off, it is what I’m suggesting.

1-Drawer 2-Door Corner Cabinet With Hexagonal Glass Inserts, MDF, Wood, Clear · More Info

You can find them cheaper than that one, with plain doors or different glass doors. You might be able to find one at a used kitchen cabinet place that matches your mantle They can be painted.

If it fits with your decor, I would put a hearth rug or faux fur pelt in front of the fireplace.

In medieval European houses there was often a ”studiolo” with cabinets filled with objects for learning, like globes, clocks, pen sets, books, reproductions of paintings to practice sketching…

What is in the rest of the room?

This is close to what the layout is like (sorry the diagram isn't great).

Here's some pictures of the room

If you have the budget for it I would build a 45degree angled wall above the mantle so you have a place to hang art- the void looks odd to me. Otherwise painting the tile matte black or grey seems like it would work well with your aesthetic.

I like that you have kind of ignored that thing in the corner and honestly I wouls continue to ignore it and do nothing on the mantel. I hate corner FPs they are always bad design IMO

Based on your room, I’d paint it all black, or white with a stencil. Could become an attractive feature in room and accessorize with a minimal look. Use various heights and skip adding anything on wall.

https://mrsmeganjane.com/how-to-stencil-your-fireplace-surround/

I have no suggestions but wanted to note how lovely your room is. Lovely restful colors, beautiful wood items.

I have a corner fireplace. We use it all the time in the winter to add a cozy, warm in-obtrusive ambiance to the room. It certainly doesn't stand out as anything disharmonious in your room. If you use it, the heat will not be good for whatever is above it. We have some ceramic and porcelain pieces above ours, less damage than a picture might get.

I would leave it as is. Unless you want to give it a makeover and make it a focal point. Even so, I'm lazy. Maybe just a plainer mantel, something more modern. I've painted fireplace surrounds, pretty easy.

On the flip side, if you are artsy, stenciling the tiles is a cool idea. Hand painted tile look at a fraction of the cost!

Even if you are not artsy you can usually find someone who is and pay them to do the stenciling and it will still come out way cheaper and be nicer than many other things you could have done with it.

If you continue to think the corner space still looks too empty, and you don’t want to drywall and just enclose the box to hang art there, if you like fabric art, it might work to simply suspend one there and the heat shouldn’t be an issue.

Dali DAL-1000 28"x19" Wall Hanging · More Info

I just came across these, and they are sizable

Coastal Brown Wooden Shadow Box Set 39130 · More Info

A couple of floor vases and another with some compass grass would add nicely to the beautiful room you have started.

Compass grass one didn't post

Pampas* autocorrect...ugh

I like the idea of this, (not so much the execution), but would try something similar with layered art (lose the word signs):

These are more nicely styled - not corner fireplaces, but you get the idea. You could start with your mirror and build upon that with art and some decor items: