I would remove the mirror and replace with art of similar or smaller size. The mirror is just reflecting a light and railing which is distracting.

If interested, it needs quite a few updates, as just accessorizing the mantle won’t have much impact. Painting the tile and mantle is not hard and you may find less of a need to replace fireplace entirely.

https://www.settingforfour.com/painting-fireplace-tile-9-fireplace-updates/

I did mine and it made a huge difference.

Then search on line for an overall look/combination you like, as your items are not connecting, either proportionality or style wise.

If only interested in the accessories, what you choose is personal, but the articles will help with balance, etc. and suggest a traditional look due to the sconses.

https://www.stonegableblog.com/decorate-perfect-mantel/

I would take the sconces out and just add big bold art. Nothing more

It would be easier to make recommendations if we saw the fireplace in the context of the room. We don't know how far the wall of the fireplace extends up and that will have some bearing on how large the art or mirror should be.

If you use a mirror I'd consider one with antique glass to obscure the image reflected. Looks like the ceiling light is too small for your room also.

I can see that the fireplace is on a wall all by itself.

By the reflection in the mirror, I can see you are a traditional style...

I would paint out that entire flat of wall up to the ceiling including the mantle and tile all the same color. either to match the room walls, or a deep rich color to accent SW Naval or a deep rich jewel tone you would like to highlight in other upholstery in the room.

remove the sconces and put plate covers over the electrical box(s)

Find a large canvas art you love,

Large and verticle to add height to the space. GreatBigCanvas.com make it a focal point.

here is a sample of what a monochromatic paint job could look like.