If you aren't moving, I'd ignore trends and get what looks good and works for you (since it'll be untrendy before it matters). In other words, I don't now what is trendy right now, so keep that in mind. This is just what I think would work and look good. :) I like what you have, with the mirror going almost all the way across. Leave enough space on the sides for hanging towels (retractable makeup mirror maybe?) so that they don't block the mirror. Possibly a wider light fixture than what you have pictured but at a similar height. I'd do a mirror with a small frame or beveled edge. (I think unframed mirrors are absolutely unpopular now, but your mirror is already sort of framed by the space and less might be better. A bulky googah frame could be overkill.) A medicine cabinet isn't necessary and won't be missed unless you like using the angles to see the back of your hair, but there are better ways of doing that. And a medicine cabinet over the sink would require multiple doors and a seamless look would be better.

I have done a box before. In my case, the light fixture ended up on the slanting part of the ceiling and the fixture could not accommodate the slant. So I had my carpenter build a box to match the slant and make a level place for the light to attach. So you can have a box made for behind the light fixture. Others have posted here, asking for help finding a light that protrudes enough, and I don’t think any have been found. Can lights have their own issues above a vanity. They usually make shadows on the face of the person shaving or doing make-up. Light behind you may help. These ceiling lights are very easy to install and they send out the light 360 degrees around, plus, their color can be adjusted. They might help in your situation. One over the shower is nice, too. I found them when doing my bathroom and loved that they need no electrical box behind them and take up only the depth of the ceiling drywall. My builder had used them before and also loved them. You don’t have to worry about ceiling beams as long as the electrician can get the wire to them. https://www.build.com/dals-lighting-5006-cc/s1763172?uid=4163442

Hi. https://www.houzz.com/magazine/how-to-get-your-bathroom-vanity-lighting-right-stsetivw-vs~57853042

I feel the mirror needs to extend higher which you can do if you use a horizontal vanity light. Here's a very rough example mock up with a West Elm slim framed medicine cabinet (suggest the larger size if it works with the cabinet) and a George Kovacs horizontal light. I'm trying to follow the same lines of the cabinet, top and bottom. Hopefully a pro will weigh in on the necessity of that. I feel you need a slim framed unit as the space is tight and the slim frame will give the eye a resting spot. Same with the light, keep it simple. Functional items in a bathroom create a lot of visual activity so I think these items should be calm. Can you raise the towel ring? It looks low to me. Nice refresh!