My preference is towards Universal Design (also called Inclusive Design). Under this paradigm I would recommend the following:

1. Remove the wall in between the bath and toilet

2. remove tub and install walk in shower flush to floor, with a bench and hand-held shower

3. Keep the wall between the sink and shower unless you need a bigger shower or want to move the sinks to a more convenient location.

4. No cabinetry under sinks. Redesign or demolish/ rebuild the built-in wall cabinet to use for storage

5. Lever-style faucets and door handles. Makes it easy with soapy hands.

6. Grab bars in shower and around toilet.

7. I think the window locations are fine. If you remove the wall I think they will become more visually proportional to the new, larger space.

8. Everything else is just aesthetic and up to you.

Note: I don't believe in bathtubs. People rarely use them, they waste water, and are more dangerous to use. I don't know how much the lack of a tub will affect resale value.

Can you draw a to scale floor plan, including windows, doors and measurements? What budget do you have for remodel? It's possible the wall between the shower and toilet is loadbearing. Have you been able to determine whether or not this is the case?