You can add a full drapery panel over the right side only, and draw it back slightly, to fall to the side of the bed.

Dress the window as if it were centered, meaning have the left drapery panel cover more of the window than the panel on the right. This will give the illusion of a centered window. Another option is to dress the whole wall with drapery fabric; it can be the backdrop to your headboard. Attached picture is to give you an idea of what I mean with option 2, although this is not a bedroom photo. Either option will work, but option 1 would be less $$ - less fabric needed. Good luck!

In the interest of extra storage in the master bath, I added one extra bank of drawers, which means that the window is off center with respect to the double sinks. I mounted a long rod that almost touches the medicine cabinets & the curtains are adjusted so that they completely hide the extra wall on the right.

A cornice treatment is a good option if the window isn't needed for light:
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Consider a shutter centered with the bed, placed over the window & extending on to the wall... it could be 4 panels across & the side panels could have louvers closed & center panels open to let light in...

I agree with Susan, that is a great idea... But no mater what you do, you will have to keep your drapes or blinds closed all the time to give "everything is centered" impression and no matter what you do, something not centered will always look off balance.
Since you have everything open already, and there is no header above, why not install a double window of the same size to make everything centered and not only it will make the room look brighter it will bring everything together... Not to mention all you have to do on the outside is trim the window out without disturbing, adding or replacing siding. Just a thought...

Good luck

In this case I think that the curtains could be partially open and still maintain the illusion. You are absolutely correct that there are times when the window is far to the right or left and you cannot.

if the room is still in the state of the photo why not just have your contractor reframe the opening so that it is centered and a little wider. By the looks of the beams it would not be a big change order. If there is no $$ for that then you have to revert to fooling the eye maybe with floor to ceiling panels that make it seem as if the window is wider.

Center the bed under the window and then add a comfy chair and ottoman on the other side of the bed. Or you could add a chest in the area to the right of the window if there's not enough room for a chair. That should keep it balanced.

A shutter on that window would add to the cottage look.

Thank you very much to all of your fantastic suggestions! The room is still in the state photographed above so the options you have put forward are all still applicable. My favourite ideas are the shutters fixed beyond the window to give the illusion of a bigger window, and also to make the furniture choices different on either side of the bed. I can't wait to decorate this room, your advice has helped me significantly! If there are any further suggestions feel free to post them!

I agree with Susan J Designs and GN Builders