What about eliminating one of those hall closets or moving the door for it over and then bumping the door to your bedroom over to the right a bit more (might mean you will have to shift some furniture).
Then you can expand the closet a bit. Not sure on the best door placement, but you could re-use the existing door and trim. I'm not sure if you'd be able to fit everything in my A version. Another idea would be the B option to extend to the exterior wall behind the other bathroom and stretch the closet. The toilet or shower could go in the small area near the exterior wall (toilet would probably be my suggestion because the plumbing would be easier to access than a shower-- which would need to have water supply in a wall-- toilet supply line can come up from the floor if need be. It is not recommended to have water supply in exterior walls.
I'm not sure if a double vanity would fit in either scenario though. So long as the toilet has at least 30" around it (15" from center of toilet to finished wall on each side) you have enough space. Shower will need at least 30"x30" (in some jurisdictions you require more). You may be able to fit one of those rounded corner showers in the space.
In option B you could move the door up more to allow more space for a shower on the bottom wall and have double vanity on the left wall, maybe.
Keep in mind that in addition to the drains for your fixtures, you will also need to have vent pipes. The plumbers here can probably help you more with the layout on those. You will have to make sure the floor joists don't interfere with any of the drains and figure out where the wall studs are as well. Toilet needs 3" or 4" flange and at least 2" vent. Shower will need 2" drain and at least 1.5" vent.
You need to think about how the new plumbing will tie in to the existing plumbing.
One minor suggestion, if you go with the toilet in the little pocket in figure B, you might want to make a light fixture that looks like a window to go over it-- it can basically be a box with a window frame and a light inside with glazed glass so it looks like a window. Also, keep in mind that you will need some sort of moisture vent for the room. Adding a window to the exterior might not be an option since you said it's historic, so you will need a vent fan that will have to tie in to HVAC ductwork or have it's own ductwork that vents outside the house. It can't just pump the moisture into the attic without causing moisture damage.
Plan C (which I did not draw up) could be to take the closet on the right and make it accessible from the hallway, make the hallway accessible large closet in to the closet for the bedroom, eliminate the smallest closet and use it as the entryway for the bedroom, move the bed over to the right wall with the foot facing the bathroom, and extend the wall for the bathroom similarly to Plan A but have it cover where I proposed moving the bedroom entry door to instead. I hope that makes sense. Plan D could be about the same as Plan C but reduce the width of the room and extend to exterior wall as in Plan B.
Does any of that make sense?
Edit: I did mockups of C and D.