A niche that fits into the studs should be ample enough to add soap, shampoo, etc. So, unless you need space for more shower items, I would not use the extra space for the shower.

How wide and deep is this extra space? What would you need the extra space for ? Towels, storage of personal, items?

I don't really understand the geometry here -- can this space not be incorporated into something on one side or the other?

I hate left over potential storage space, but this was probably done because the current closet space is very narrow and if you make shelves too deep you can't reach anything. You don't want to make the current space deeper - will be terrible for storage. What is on the other side of the walls? Could you open up drywall in a hall or other room and create some storage?

The best use of the space would be a pull out shelves type configuration. Look at Ikea deep pantries which come in 24 inch depth and 15", 18", and 24" widths and can be configured with full extension pull out shelves or drawers. Those hold a surprising amount and you can add your own custom door if desired, as well as trim to make it look built in. Anything deeper will not work unless you have full extension pullout shelves for the full depth but then you are talking custom and probably twice or more the cost of the Ikea ones. https://www.ikea.com/us/en/cat/high-cabinets-23609/?page=2

@ scrappy25 - I didn't think about pull out shelves - great for narrow deep spaces.