Enlargement of the bathroom in the attic including the sloping ceilings

Yes, folding door should work, I also have one on my asymmetric one. And I find it practical too.

The remaining hallway up there, does it lead anywhere or would something like that be conceivable as well?:

The hallway no longer exists, we removed the wall from the bedroom to enlarge it The 2nd (red) wall also no longer exists, so even before we bought the house, but it will be rebuilt