The most clever bathroom storage I've seen was where the wall directly in front of the commode (you're looking right at it when sitting on the commode) was hollowed out and used for storage. The contractor cut a roughly 2ft tall x 16in wide section out of the sheetrock (basically, the space between two studs), put hinges on the side closest to the perpendicular wall that was about 12in to the left of the opening that was cut, put 2in trim around the newly-created door, and made the whole thing into storage for toilet paper, bathroom cleaners, etc. BTW, this was done in an exterior wall.

I had cabinets built in above the toilet in my bathrooms.

Not a trend-but I found it very helpful to buy and install the longest "closet" I could find to fit in between studs. Wayfair sells one unfinished (I mean plain wood, no stain), and then we asked to paint it the wall color only in semi-gloss). It dramatically increased storage wihout being in anybody's face.

But something like towels wouldn't fit in there..it's good for spare shampoos, medicines, whatnot. Towels are on the hooks -we have a shelf with hooks, left from previous owners, it's more vintage-y than something on trend, but I decided to re use it. It's across the toilet, next to the door ..one can't see it unless one is inside.

spare towels are in the hallway closet.

Very small space-a cabinet over the toilet. I pick these second hand because that's what I like..but that depends on your and house's aesthetic of course.

One can also hunt a mirror with a shelf. To increase storafe a bit (again, not for towels, obviously) Not easy to find a good one but possible. They come in different designs-can be contemporary looking or can be more ornate...