Factions shouldn't even matter for new expansions. Players on both sides are privateering murder hobos against whatever the "cosmic threat of the year" is. Just let Alliance and Horde players group up and communicate in expansion areas. Just like the Commonlands in EQ; the evil races went to Neriak for stuff, and the light/neutral races went to Freeport/North Freeport. You go to your faction town, sell your shit, and meet back in the field to work together again.
We've partnered up so many times it's stupid to keep it apart; no, an Orc shouldn't be able to walk into Stormwind, but in the Shadowlands, who gives a shit if Humans and Orcs work together?