Congrats. You have done a beautiful job so far. We consult on about 30 to 50 kitchens a year. Nice to see one as fun as this. Couple of points. Love the backsplash behind the stove. But any grease pops will hit the wall on either side. What if you do the white subway on either side of the backsplash you have and also on the sink side. It could be beautiful and functional. And since the tiles you have now are aligned, we would do the subway in a running pattern. (staggered) Love the visual idea of open shelving, if its fits your style and needs. And would love to see a little wood brought on to add a touch of warmth. Just look at the bit of warm color in the door mat, and imagine some of that warmth on the walls. And as far as the subway tile, it is totally fine to mix in a bit of traditional with the more contemporary pieces. This keeps the space eclectic and fresh, as well as more unique. Great job so far.

If you need the storage I would probably do white wall cabinets on the sink wall. If you don't I would probably not do anything else on the sink wall. I would extend the current tile on the range wall to the refrigerator and the corner, but it looks fine as it is.

I don't think I would do open shelves. It's very clean looking and I think it should stay so. No microwave?

would a thin dark grey or black grout line be ok with the subway tile?

Microwave is on a pull out shelf in pantry to the left of fridge. Used minimally for warming up stuff mostly so it works for me. Didn't want one of those hood microwaves and didn't care for having it on the counter top. Right now the storage is fine so hiding it in a cabinet has worked out nice for my situation.

How nice (uniform ) is the stuff that you would put in glass front cabs? If it's not gonna be as pretty as the images you see, then white modern, euro-style cabs might be a better choice.

Subway tile with open shelving might look very nice.