For me it would be an MZ-S with the following changes.
1. Slightly bigger viewfinder and higher eyepoint.
2. Preferably an interchangeable finder.
3. Faster AF and more accurate servo AF.
4. Slightly bigger grip closer to the Z-1P's gip (with at option to add a handstrap).
5. Faster frame rate say 5 fps would be enough
6. Quieter operation
7. Metal back (or something that feels more robust).
8. Improved exposure data imprinting e.g print actual exposure time on Bulb exposures.
9. Higher flash sync 1/250
10. Built in flash exposure compensation. available in all modes. with a separate compensation scale / indicator in the viewfinder.
11. Exposure adjustments in 1/3 stops. Even it this requires aperture control using a thumbwheel on the body.
12. And last but not least .... (Digital option via a digital back a' la Leica R9) (Dreams dreams dreams!!!)
A camera like that would not just be a film swansong but it could serve as a platform for future DSLR development and may even make commercial sense.
Patrick