Below is about the only thing I've come up with that I don't hate too much. It's surprisingly similar to the current layout, maybe there's only so many ways to lay things out in this space.
Somehow, we'll drag the whirlpool hottub out of there, and build a knee (shoulder?) wall at about the 6'6" height and put a double vanity against that. There's just enough room for a 22" vanity to fit between the wall and the window. The toilet stayes roughly where it is, it'll get pushed over an inch or two to ensure that we've got 48" for the shower. We had hoped to get one of these for a shower, but it seems like it will work less well up against the toilet. Instead, we'll build that as a solid wall and build a custom tile enclosure. We'll put a sitting tub next-ish to the vanity. I could only find a model for a 6' tub, and we'll be using a maximum 5' tub, so there will be much more space then it actually looks like. I'll move the knee-wall in as far as possible to allow the tub to be in there and put builtins into the knee wall.I dunno, maybe this works. I'm not totally sold and keep waiting for an epiphany.