If you made the tub area the new walk-in shower then you'd have your drain and water supply in place. The angled wall would just be part of the shower.

Turn the old shower into a closet.

Ptreckel is absolutely correct. A good designer can make your bathroom. The corner is probably empty, but it could be hiding plumbing or something else. If that space is critical, I suggest you open a hole in the wall to investigate.

Kirk, MVH