I agree it needs something. A blind would be a problem IMO because if you hung it within the window bit then it would reduce some of the light and make it probably look wider and shorter. if you hung it above the window then it would make the ceiling look lower I think when open. so I would say you might have to think about curtains. you could get curtains made for the actual size it is but there is no reason why you couldn't get them a bit longer and then when closed it would make the window area look bigger. not sure what that would look like when opened though.
I would go for roller blinds actually, just above the window. IMO curtains are going to look weird and not fitting with the style of the room. In all honestly, I'm not sure you need anything if you don't need it for privacy. I would put something under the window though, a console table maybe with photos on the wall, lamp, art, what have you. It is a nice room though, so I'm sure what ever you choose will be ok.