Yah, now that I look at the diagram, this space is huge. Turning into a single bedroom might not be optimal. Upstairs, 10x10 or slightly larger is probably enough for a BR.
I agree to with the suggestion to not do too much......as a buyer, I never like to pay for someone else's upgrades.....or their style An extra BR is worth it, but a buyer would rather choose their own "style". i.e. make it a BR, but don't personalize it in any way.