If I'm not wrong, priests mainpulate / channel the Light, while paladins are infused by it. That's lorewise.
Now, if you look closely, you can see they want the number of clases available the same on both factions (5 highmountain , 5 lightforged, 7 both elves).
Nightborne paladins make no sense lorewise, so that leaves highmountain (and IMO, that 5 fit better with the "commune with nature" of highmountain tauren) or zanadalari (much more likely).
My bet is:
Zanadali: warrior, hunter, priest, rogue, shaman and, maybe, paladin.
Dark Iron: warrior, hunter, priest, rogue, warlock and, maybe, shaman.
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Highmountain culture is much more close to Night Elves that the normal taurens. They were blessed by Cenarius, and worshipped nature and Elune since then.