Well, if you don't want to move anything...

I'd put a shallow linen cabinet in the 55" section of the window wall (maybe a 36" one, or whatever seems comfortable when you mock it up)

And I'd probably put another one on the 115" wall, maybe larger.

That is, if you don't want to move the toilet, tub, or window.

However, if you don't want to move the toilet, or tub plumbing, but are open to changing them out and the expenses to do so...

I'd turn them each 90*--toilet facing the tub. Tub now along the long wall.
Then, a larger linen cabinet in that 55" area that also privatizes the toilet. And, another at the end of the new tub, near the door...

What about just moving the vanity and adding some walls. Something like this, though for some reason I'm having a hard time figuring out the scale right. I need to get off the Internet and rest my eyes, lol!

Blue stuff is a privacy curtain and towel rack/hooks.

Oh, I like that. I can't wait to show my husband.

I didn't think if the privacy curtain, but my suggestion was going to be along the same lines -- use that big open space. If you don't need that large a space for linens, you could create some bookshelves, a pair of closets (one ofr linens and one for other stuff), or possibly a desk area and a linen closet. Makes a lot more sense for a family with kids.