Of note it would be challenging to make it a closet because the door would open on my tub which is in front of that

a nicely trimmed out open storage, the bottom could be just decorative if you can't open it.

A linen cabinet will work well in that space.

I would just use it all for the shower and have space for a nice teak bench.

Floor to ceiling drawers?

Nice thick open shelving, store towels and maybe some pretty jars with bath salts etc. I nevver design a space without thinking of what I need in the space you can't have a door then open or as mentioned drawers at the bottom you can actually use and then a couple of shelves