It almost looks like they kept that area open for a towel bar. I would put in a towel bar, art above the toilet and maybe a decorative box under the towel bar that has a lid. I have a box in every bathroom, fairly close to the toilet, that holds the extra toilet paper rolls.

I love the suggestions of statement sized horizontal art. You don't need to fill the wall, just make it interesting. A suggestion for your fabulous new built-ins: if you can move those shelves around so they're at different heights. Not every shelf needs to be used. That way you can put something really tall in one of them.

My suggestions would be to have a piece that has height. Iny sketch it shows a console table, could have a lower shelf or not. There is an art piece or mirror hung above with a small plant or candle holder accessory with a small stack of interesting books. A tall plant like a palm that was already suggested would also be an easy accent piece.

The wood carved panel you selected are nice but they may look too big for the wall where you intend to use. How about this choice? You may need two of these. https://www.etsy.com/listing/531538634/teak-live-edge-mirror-medium-b076?gpla=1&gao=1&utm_campaign=shopping_us_Tropicalwallart_sfc_osa&utm_medium=cpc&utm_source=google&utm_custom1=0&utm_content=14080657&gclid=CjwKCAiA8rnfBRB3EiwAhrhBGis-cxbwQ99y3lyNPZF1bjkWnJsyavOmi69GYHgeC-HP5161xWaJFBoCHRwQAvD_BwE