I think the kitchen looks great. Perhaps a pile of chopping boards next to the sink and something vintage on the shelves. I wouldn't do a coloured splashback with an industriał look. If you really want to change it and budget isn't an issue (they are dear!) try an antiqued mirror splashback. You can also just silver leaf the ŵall and then have a clear glass splashback for a budget version. Alternatively have a look at metal sheeting splashbacks, you can also get hammered ones etc for texture. If you don't mind mixing metals then try bronze or copper. I had thought you could do american tin tiles on the ceiling as an alternative to bounce light but then realised how high the cupboards are fitted.. You could also look at tin tile wallpaper and again clear splashback over the top. Not for light, but leathered marble looks great but I'm struggling to find any in the uk

I would either add a pattern of some sort (examples in my splashback ideabook) or a mirror, as it would tie in with the grey scheme, but add more light to the door fronts and appliances!

Cool kitchen! I think a coloured splashback would look great, we have a great selection of tiles - metro tiles would certainly suit the theme. And we have new Lustrolite gloss panels which can be cut to size which would also work.

Lustrolite Carbon Black High Gloss Wall Panels · More Info

Antiqued mirror would look great with your grey theme; as you said it would reflect light around the kitchen and in turn make it look bigger. You could use metro tiles or one piece of glass, depending on what you prefer. As you mentioned, because you have quite a busy feature wall already, I think it would look good using a subtle antique finish. Let us know if you would like some free samples to look at.