I really like that last photo that celerygirl posted! I can see that style in this room very easily. Are you keeping the wall color the same?
I think a wrap around desk going left from the window could be a nice addition to what you already have. It would be nice if you can find a space there fit in a reading chair with lamp, if that is something you would use.
I've seen some really neat things done with Billy bookcases, where people have made them look like built ins.
I have a similar space in my house that was actually designed to be an office; unfortunately it has no closets. It has a built in oak desk at one end, a narrow window above it, and a cabinet (like an upper kitchen cabinet) on each side of the window. I use that a desk and fabric cutting table. On a side wall, I have almost the exact same IKEA desk set up you have. And I also have an Kallax (formerly Expedit) for fabric storage.
https://www.thespruce.com/ingenious-ikea-billy-bookcase-hacks-4006865
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