Media rooms are often dark so the movie theater ambiance is enhanced. However, I think gray is on its way out. Work with your sofa color instead; it looks to be more on the cocoa/chocolate side. You might consider that teal like your cushions and think out of the gray box. Your insp pic probably has WP or a faux painted wall.

We need more pictures of the room. Will the fixtures be in the same spot? What color will they be. Are you planning on tiling behind the vanity as in your present powder room or are you switching to a vanity with a framed mirror as in your other photos? I would paper only one wall - the one behind the washbasin, if you are going with the pedestal. Then paint the other walls a lighter shade of the same color. Both of the papers you showed are lovely, but sorry I disagree with your decision to avoid blue because you have blue in the kitchen. I think it is more harmonious to carry colors over from one room to the next. So if you really want yellow or green, you might look for a paper that has a bit of navy with either of those colors and then use navy for your towels and other bath accessories. I do agree with ILoveMod that the floor is too earthy and rustic for the paper. If the flooring is also in adjacent rooms, I would stick with that flooring and look for a more muted, olive green. Below is the kind I would look for. And then I would get hand towels in that muted green and navy. You really need to get a little sample of the paper and hold it against the flooring tile and see it in the lighting in the powder room. Of the two papers above I prefer the top. If I used that I'd consider painting one small wall or a soffit or even the ceiling navy. I like the middle one a lot too. With that you could paint the walls yellow, but not too bright. Good luck with it and please post again to let us see the finished room.

I’m loving Abnormal Anonymous’ whimsical wallpaper designs. https://www.abnormalsanonymous.com/products/mr-blow - they’re known for this cheeky blowfish for example. Also, Joss & Main has sophisticated options such as thisCandice Olson design: https://www.jossandmain.com/wall-decor-mirrors/pdp/candice-olson-breathless-pavilion-33-l-x-205-w-wallpaper-roll-whw4811.html?piid=28907799&utm_source=pinterest&utm_medium=social

I’d look for a smaller, better proportioned mirror and a rectangle shape would better suit your sink. Tape out proportions before purchasing. Now you have room for waterproof sticker tiles, adding one or two strips across sink. The least money/effort, but solves water issue and creates a ”look” which you can expand on. https://wood-insider.com/a/s/products/tile-stickers-dutch-antique-24pc-6x6in-peel-and-stick-for-kitchen-and-bath-bw002-6-24pcs Add new art over toilet for more impact and to feel better coordinated. Don’t forget lux towels for your guests.