The Ren'dorei via Magister Umbric were featured regularly in Vol'dun and Zuldazar alongside the Explorer's League and against the Reliquary, so I wouldn't be surprised if Umbric appears here again. He appeared out of nowhere in Vol'dun when no one expected him, after all.
I like this new development actually, we always talked about this grand role the Void elves had in fighting the Void, but at the same time, I'm also down for more fun and light-hearted stuff, like Umbric accompanying the explorers on an adventure. But we also know that Alliance and Horde expeditions will be working together, so I would actually like if Umbric interacted in a friendly but competing way with the Reliquary, I know there is a story to be told here and I know the story will be good.
We know there will be a Void Lords expansion anyway so there's no need to rush.
Besides, wouldn't Magister Umbric's exceptional skills come in handy here? He can create portals wherever he wants and to whatever destination he chooses, it seems like this would be of great help for the explorers at the Dragon Isles.