I like the idea of a thin stone slab similar to a countertop especially a darker granite. I would be impactful is you took it floor to ceiling. Might provide a very cool focal point.

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The room has such a lovely symmetry and view so I wouldn't mess with that by going with anything too strong. My inclination would be to match the pale tones in the room and create an almost seamless feature wall that looks like it was always there, and doesn't compete with its surroundings too much.

heres just a few ideas, but basically look for a something that just carries on with the vibe you have of cream, pale wood tones and warm grey.

Best of luck