Since he's also the only PW who currently is at least partially white that I could see also being partially black, unfortunately, that probably means that, barring a new character, the odds of an Orzhov walker are fairly limited. Out of currently-existing PWs, barring sudden color changes, the most likely in my pattern are Kiora, Nissa, Sarkhan, or Tezzeret. Technically, Ajani could, too, as he often gets counted for green. Bolas is not terribly likely due to story reasons, and Karn even less so.
Out of the first four, I'm... kinda hoping for Tezzeret.