well, except for Nightborne, it's based off of rep in Suramar, so it doesn't matter which faction you decide to use, as long as your Suramar (Nightfallen) rep is exalted. i would assume that the same would go for a few other races. If i recall reading correctly, Highmountain Tauren would be unlocked with exalted HM rep. Argussian Reach for Void Elves. as for the others, not quite sure. but given it is this early into everything, not 100% on the sources.