I would start from scratch and get rid of that tile first. It doesn't really belong. Too much texture for a full bathroom. You don't want moisture from the shower to leave the possibility of mold and mildew to grow in the crevices.

This wall is in a nonwet area and will stay .need to coordinate something else to go with the travertine tile on the other walls

Even a "nonwet"area will be affected by steam. Also, that texture behind the sink and toilet is very hard to clean and sanitize. If you’re set on keeping it, I’d get a color wheel and match the darkest color, and find a solid, large format tile to match that. Might as well let the travertine be the star.

Darkest? How about lightest...caves are over rated.

IMO that travertine wall is the worst thing in the space . Is this a space near a bedroom? I see no point in a shower or tub in a PR not serving a bedroom nearby. A to scale floor plan will also give us some clue as to how you plan to add the shower . Post the lan here on graph paper showing every measurement door way and window all clearly marked posted in jpeg format in a comment then you will get real help. DO NOT start another post . If you strat with what you are not willing to do you get limited help

Simple tiles coordinate well once you have a stone wall playing the starring role. Look at tan and beige tiles from American Olean and Dal Tile. Their classic lines have several sizes and shapes, plus all the trim pieces.

A good exhaust fan will minimize humidity from the shower.

https://www.daltile.com/tile-product-category

I would go with a very simple tan to match the lightest tone in the travertine.

I’d also bring home samples of several light tans— not the deep apricot of the wall shown. Something that standard white toilet or other white porcelain fixtures you might be using still goes with, since they are classic. The exact “ tan” might depend on what kind of tile, its texture and undertones and what you need, so that’s why look at a range & see what jumps out as wrong or right.

Then if get a good light- to medium neutrals, you will be able to pick towels & things— ranging from white to beige to pumpkin , depending on what you like. A green plant ( a real one that tolerates lower light) would look fresh with all the earthy tones— if there’s room. Or wall art.

I would try to lighten it up. Something like this

Take the lightest colors out of the tile . Keep it a simple pattern. Even go with large format tile so that is stays " quiet" since the travertine is so busy. You could even do the floor in the same tile like shown here.

Good luck!

There is a study adjacent to it that has been converted into a bedroom and now adding a standing shower and modifying the space .