Mer and Alex as a couple
Hear me out here. I think I'm in the minority in saying that I would really like to see Meredith and Alex at least give it a go as a couple - i.e. George and Izzie in seasons 3 and 4. If it were up to me, I'd pair off Jo and Deluca, Maggie and Riggs, and Mer and Alex so that everyone can have their happy ending.
Plus, I think it would make for a really interesting twist if Shonda sort of pulled the carpet out from under us for once: all the well-established couples actually don't make it and everyone moves on with other people.