Please post a pic here in the comment section so we can see what you see.

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Put in above search box: mantle decor in photos, and see all the great ideas!

I don't think a mirror is ever a bad idea, but that one looks too large. I think a round 26-28 inch mirror would look nice with two higher items flanking each side. I change out my mantle decor seasonally. In the fall, I have two tall wheat bundles, at Christmas, I have some candlesticks with garland, Spring I have some Boxwood topiaries, etc. Keep it simple.

I am serious when I ask why is the mirror not good / dumb? do you like the frame's design [I cannot see it]? add what you find interesting or pretty...

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Because you have plenty of shelves for many items, maybe instead of adding to the mirror there, one item? this I haven't seen in person: W 48" / D 4" / H 19" / 14 lbs. curved steel.

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If you like to bring out the delicate colors in the bricks, this type? one on each side... or another one.

This is nice, do you think so? you can add to mirror. These folks kept a simple collection at the sides: plates, and I doubt the room looks messy

1/3 books 1/3 stuff, 1/3 space

Large items on bottom smaller as you go up

Picture frames in one or two finishes.

If you do decor items keep style color finish similar.

Say you had this simple tiny repeating -- maybe you could add color in collection at side, of larger:

Frame a bit rustic like ceiling beams, and delicate like upholstery...

Turn the mirror sideways.

Mirrors are supposed to reflect something nice... if yu don’t see anything nice reflected in it or cant see yourself in it what’s the point? I see your ceiling reflected in the mirror... We used to have a mirror.. instead we hung a beloved picture.. now we see it all the time... much nicer..

I like the photo from everdebz, and the suggestion of 1/3 each books, decor and space from Susan. As Lisa mentioned, art would be a nicer choice than a mirror in this case.

I think my mantel reflects my cluttered life. I still have my Christmas nutcrackers and angels up. Hey officially tomorrow is Epiphany the 12th day of Christmes. I have an array of sentimental gift porcelains that I rotate with candles to fit the seasons. I have a pair of hurricane lamps that in recent years have held those battery operated mini lights and Christmas balls. In summer I put my seashell collection in them with candles. I may replace those in the fall with the acorns I need to sweep off the patio now. Your mirror fills the space so just a couple tall candle holders or hurricane lamps is all you need. Then you can add some seasonal color if you like or keep it simple.

You might experiment with things from your shelves. It doesn't have to be perfectly symmetrical. Think of it as a see saw balancing similar size items on either side. Those plants in white pots would contrast with the dark brick nicely. The mirror can reflect the light from candles. (Some of those battery operated pillars even have timers so they glow every evening and go out at bedtime.)

I change mine periodically, but right after Christmas decor went down I put up one large painting, and left it at that. After several weeks of Christmas decor everywhere I was ready for something clean and simple!