I personally wouldn't add anything. Looks amazing as is and I like the glass backsplash behind the cooktop. I'm sure others will have good suggestions for your kitchen.

Nice modern look! One thing I notice is the lighting, especially the undercabinet lighting, is casting a warm yellow color. The upper cabinets look to be a dark grey. But the lowers look more dark green on my ipad. Is that correct, or are they all the same grey color? The white quartz even looks yellow in some spots. I think it will be hard to choose the backsplash unless you change the bulbs out to something with a more neutral white light. Not too cool though or then it will make things turn more blue. Hope that makes sense. 😊

Rebunky here is better picture

I think continuing the countertop as backsplash would be in line with the style of the space.

If that isn’t an option, match the tile to the base of the countertops. And you can go with any shape.

Picket, shell, stacked subway, vertical subway...