We have travertine on the floor in two bathrooms, had travertine in the entryway from the garage, and only took that up when we tile the entire downstairs with a different travertine (re-tiled it for continuities sake, not because of wear). Yes, it can scratch or chip (our entryway tile had some minor scratches, but we've tracked stuff in from the garage, have slid boxes across the floor, had workers coming in and out through the garage while reno'ing various parts of the house, etc.). But, scratches and chips on travertine can often be repaired, whereas ceramic/porcelain tile cannot be. I have buffed out etch marks (using a green scrubby) on the honed travertine in the guest bathroom (former cleaning lady set the bottle of TB cleaner on the floor and it left a nice oval shaped etch right in front of the shower). I've used a tan/brown permanent marker to "color-in" scratches in our travertine as well (color, wipe off any excess with some paper towel or buff with some fine steel wool), and they disappear. Like allison0704 said, use a good sealer, and choose a decent quality stone, and you should be fine.

OK, I've reread all(I think) of your posts now and I see your point about the wainscotting not really being truly visible in such a small space. You have options, but I really think the wood plank is not the way you should go, it is very far from what was your original inspiration. I am not sure it will make you happy. I think I would do the wainscotting, scaled down to fit the space. Layout/draw specifically the layout for each wall segment, the 36' toilet wall, the long wall next to the toilet, the sink wall. This is homework that will make the difference. The marble inset band needs to be fairly wide, maybe 6", it would be just at the level of the sinktop. Carry the marble band around the pony wall endcap, you could continue it around the shower, but I think you said you didnt want to. Top the pony wall with a marble sill. My only doubt in the cohesiveness, the peacefulness of this plan, is the two cabinets you want to put over the toilet(assuming I am remembering that correctly), I think they will make it heavy. So I would want to hold off on those until the space is complete and then decide. I think a chrome(I am assuming your finish is chrome) train shelf might be better to hold towels, yes it will not provide as much storage but perhaps it could do. I think this gives you the classic look you were inspired by.

Sativa has a good idea for a high contrast design but I might be overwhelmed too much dark black. It would need the balance of lighter floors and the natural light from the windows which could make it stunning. I bought the same pebble tile at our local H H Restore to re do our tiny shower. I want bright white tile like your subway tile but hubby still wants a large format porcelain. The mix from the pebbles can work with many things. Remember when it is sealed the colors will deepen to what it looks like wet. You could use a midtone like the charcoal grout for your floor tile if you are keeping the walls lighter like a warm white, tape or grey from the pebbles. If you go with dramatic black walls, you need a lighter grey ,taupe or warm white for balance. With either color scheme choose a basic floor tile with a bit of texture to prevent falls. A rectified porcelain with touches both cool grey and warm whites and taupes would work with lots color schemes. Keep in mind you may change your style in a few years. So pick a classic with lots of options. It will be easier to change a dramatic paint color than a busy tile pattern.

@berlinrl, I‘m so glad you reminded me about the elevator... I had it on my list to discuss with the builder today and then forgot all about it! 🤭 The builder showed me an elevator he did for someone else and I told him to copy it. The owner had upgraded the elevator interior to match her foyer... drywall with the wood trim. Just by coincidence, her house was very similar to mine. I hadn’t thought about the wheelchair bumping into walls then. The elevator is not installed yet, so I can still make a change. BTW, the owner of the house was so gracious and her house was so beautiful. We’ve since become very good friends! In our first floor study bath, we put shiplap on the wall behind the pedestal sink and toilet— it goes up to about 5 feet.. It was installed when I went to the house today! Today, we decided to shiplap the opposite wall 3’ high, to deal with the wheelchair. In the master bath, we decided to add extra trim - in addition to the normal baseboard. Next time I go to the house, if it’s done, I’ll take photos and post them. Have you had any problems with the wheelchair on wood floors. My DH got a motorized wheelchair — it repositions him etc — allows him to stand in it. It weighs a lot (at least 200 pounds) and he weighs a lot. He’ll only use it on the main floor (the elevator has a 500 pound limit and I don’t want to get near the limit). I’m thinking of engineered 7” white oak. .. I liked one product from a local mill that would be finished by my builder on site and one floor that is pre-finished by Shaw. The Shaw Floor has an aluminum oxide top coat. The flooring consultant was concerned about putting that much weight on the Shaw floor and was concerned about the finish holding up. She told me she contacted Shaw to see if they would stand behind their product for such use. I understand it is really a problem to re-finish a floor with an aluminum oxide top coat. If I get the locally milled floors and have the builder put a strong Bona finish on them, I guess they can be sanded and refinished if they get scratched up.