Your fireplace is gorgeous.

What you've got going on isn't bad. Generally it seems like people either go super heavily ornate, or super sparse minimalist on this style. Which suits you more?

Above the fireplace you need balance (which you have) Usually 3 things or 5 things - a centered object and something on each side.

In your case you have placed a centered object with a sconce on either side mounted on the wall. It doesn't really leave a place to place something on the mantle.

I kind of agree with you that the centered mirror is not the best. This is such a lovely fireplace and I want to see something rustic that complements the beauty of the fireplace. I envision a painting with a thick rustic frame and maybe a large vase on one side of the mantle and 2 smaller vases or smaller sculptures on the other side.

Go antiquing until you find those perfect couple of pieces that deserve to share the focal point of your room with this fireplace.

If this were my fireplace and I had found the perfect pieces to place on and above the mantle I would either choose a variety of colored glass bottles or a collection of white vases and a huge white water pitcher and bowl or maybe a collection of gas lanterns or big candles to add décor to the inside of the fireplace

Here are a couple of ides.

A tobacco basket. The style of this house might be more rustic than you will like, but without the dried grass etc the basket on its own is a nice simple geometric form.

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Art with a couple of stacks of books. I like the idea of an oil painting; I would lean it. Black and white photography would be nice too though.

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I like the touch of gold on the hearth in the bottom pic.

Really nice fireplace.

Less is more: avoid clutter. Don't add stuff atop the mantel just because there's a place for "something".

Mirror isn't a bad choice -- mostly because the frame is a great style -- makes me think of a flower, perhaps a mum. I'd be tempted to remove the mirror from the frame and cover it with yellow fabric before putting it back to see if I could complete the "flower" look.

If you're adding a vase, consider one shaped like a flower -- a vase that could stay atop the mantel whether or not you had a fresh cut bloom or bit of ivy in it:

https://www.pinterest.com/pin/287386019948250444/

https://www.etsy.com/listing/591382343/murano-hand-blown-glass-blossom-vase?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=flower shaped vase&ref=sr_gallery-1-10

A quatrefoil mirror is a design that is more appropriate for your Spanish style fireplace.

Certainly nothing whatsoever to do with landscape design and doesn't belong on this forum.