Moulding on the wall is a great way to create interest on the wall like this. Also consider a gallery wall. I think the polka dots would work best in a children's space, but of course your house is your castle! Do what you love.

Is the ceiling cavity above the bathroom available or does it have living space or similar above it? If it is free I would most certainly recommend a "solar tube" type solution - that is going to full your room with natural light during the day. As you have rectangular tiles that are off centre from the vanity, I would go with a large square mirror with a silver frame and I love the back lighting of the mirror as was suggested by another person but is this possible now that you have completed your tiling. Alternatively you may wish to go with a cabinet / mirror to allow for more storage of smaller bathroom items that you use daily.

Just a suggestion but have you thought about a deeper grout to showcase each tile a slightly deeper grey to the tile will showcase the patterns so they don’t get lost into one another.. Plus so much easier to clean! If you’re afraid to go too dark then go two shades darker than the tile.. Either way will work.. but as a professional having used similar tiles I always like to showcase with a deeper grout.. Just a thought!!

I would say light grout to highlight the feature tile and tie in with the tiles next to it, as dark will make the penny rounds recede a bit. Alternatively, dark grout would give a more moody feel - if this is what you are going for?