This has a lot of potential. I’m seeing it as a statement/focal wall. I found some pics for inspo and a good design will make it feel not so “off”. Depending on how big the lip is in front of the fireplace, maybe a basket with blankets or a thin book/magazine rack. It’s all about balance, right, so I’d put one large art piece or a few staggered smaller art pieces on the left side of the mantel then maybe a long ceramic-esque statue on right side (the first thing that popped into my head was a shark statue, if you’re into the nautical theme).

Love all the previous comments, I think some layered images and cute nicknacks and/or plants would be adorable

You might be able to try something like this over the right side and then maybe try a plant on the left or a piece of 3D art. Below are a few ideas from our designs.

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