For high ceilings, the general suggestion is to use all the space that you can. Your fireplace style looks traditional, so find some pieces that complement the style of your decor and take them to the top, would be one idea. You might look at hotel lobby decor to get an idea of very tall ceiling spaces.

So possibly a big art piece and then decorate the mantle?

Well, or several art pieces. I have high ceilings also and have done a little research on this topic. A couple of my favorite articles with illustrations are this one: https://freshome.com/inspiration/how-to-decorate-interiors-with-high-ceilings/ and this one: https://www.forbes.com/sites/houzz/2016/08/09/10-decorating-ideas-for-tall-walls/#5eb0a8da528d You can Google or go to Pinterest for more ideas. Look for "high ceilings" decor topics.