One of them already returned. Had too look up the name, but I remember the story... A seamstress (tailor) in Allerian Stronghold who dreamed of becoming a ranger / farstrider.
http://www.wowhead.com/npc=18390/roseleth
In Legion, she's found in Hunter class hall.
http://www.wowhead.com/npc=103427/roseleth So she did leave Outland to work with the Unseen Path, I didn't find if she became a hunter herself or just helping them with outfits as a tailor. When I found that fact I felt happy for this little reference.
You'd wonder what happened with the rest of Outland crew, since for example Danath is back (he used to be stuck in Honor Hold, but in Legion he's in Warrior class campaign and later on in Dalaran's Alliance inn). Khadgar is obviously on the front page for quite some time, since WOD. High Priestess Ishanah was the leader of Aldor faction in TBC, but we see her in the Priest class hall and later on for everyone as an innkeeper on Vindicaar.
So obviously the characters who populate Outland during TBC timeline are not stuck there forever. Return of Alleria could indeed give the remaining High Elves a reason to band together, if they cared to. Only question is, why didn't they join Vereesa before, in that case, since she's present in action at least few times like during Isle of Thunder storyline in MOP.