A reddit post recently pointed out that between WC2, when Ner'zhul tore apart Draenor with a bunch of random portals, and TFT, where Illidan closed those portals as demons were being pulled into them, there was plenty of time for those portals to have been linked to non-Legion worlds, where WC2 Horde/Alliance forces and other denizens of MU Draenor could've escaped the planet and gotten stranded before it completely became 'Outland'.
It's a shame we didn't see any such groups in Legion when we were porting around to glimpse a bunch of worlds, but it'd be a good opportunity to find long lost forces that've been largely unchanged by the passage of time, providing a glimpse into the older versions of the factions, OR a way to see versions of the factions that have developed in completely different ways.
Fun example thoughts:
A Bonechewer clan offshoot that ended up on a barren rock with nothing but demons, and developed a form of demon-hunting magic from having NOTHING TO EAT BUT DEMONS.
Dwarves and gnomes from the second war that escaped into a portal alongside Broken and Lost One Draenei and developed a specialization for stealth and survived on a desert world with giant sand worms.
Forest Trolls who fled into a portal along with Sporelings and turned into Shroom-Trolls when the sporeling's melded with the forest troll's moss on a new world.
High Elves and Humans who got into a portal alongside Arakkoa and developed into a new society on a planet where the trees are as tall as oceans are deep.