One thing I'm curious about is whether you can unlock the races fire the opposite faction... At least for the first four. Currently I only have one level 110. We don't know how the unlock will happen but most assume it will be via rep or after completing their quest line in Legion. The thing is both of those are available to both factions. Alliance gets rep and quests with Highmountain and Nightborn just as much as Horde does. The same is true for Alleria's quests and the Lightborn. Hopefully it will work for both as I really don't want to have to level a Horde character all the way through Suramar again.
Also, I know nobody knows yet. I'm just curious for thoughts.