Shower is a no-brainer in terms of "good idea" but it's already plumbed. The most sensible spot for a shower is in the back corner, but to do that I would have to dig up the entire floor to move the toilet, then move the sink plumbing, then install the shower drain.

An alternative would be putting a shower in on the other side of the vanity and expanding the bathroom in that direction. That would be cheaper but then would probably need to move the bathroom door opposite the toilet which I think is kind of awkward. Thoughts?