What I did in my home, I got a big mirror that fits between the walls going back on the mantel to leave as much surface as you like in front to dress it. Photos, a lamp or what ever you like.

What do you have? Do you have any candle holders? Art? Vases? Sculptures?

The mirror idea is good.

You can use corner shelving to place items like vases and sculptures and minimize the clutter on the mantle.

How about a nice painting on a table-top easel? Too many small items will look cluttered.

I don't particularly care for this setup, but I think something large and round would like good with candles on each side.