Canonically, all of the Zul’Something dungeons and raids thus far have been cleared by the Horde, so I don’t really think we should focus on which faction and its races have done the Trolls the most damage.
It is besides the point, really.
I really think that if Blizzard wanted to bring all of the remaining troll tribes under the Horde, BFA would have been the perfect moment to do so, by far, but, alas, they missed it. They could easily focus a part of the Zandalar campaign or the story of a later patch on Talanji uniting them under the Zandalari Empire and then bam! Shadowlands and its new customization hits and makes all of them “playable”.
With WoW going full in spaaaace now, I really doubt that we are gonna see any story progress regarding more “earthly” plot bits (such as racial stories), regardless of race, despite being sorely needed by each and every one of them (yeah, even the Humans). I fully expect all of it to remain in limbo for a lot of time, unfortunately.
There is nothing up with the Dark Horde after it got defeated along with its master, Nefarian, during vanilla.
Given that the Smolderthorn despise the Horde-aligned Revantusks for being, well, Horde-aligned, I doubt that they would join, unless in case of a lolretcon, which could always happen since it is “old” lore. The Firetree are fair game, on the other hand.
I don’t really think that tribes are an issue, however, but whether Blizzard wants forest trolls to become playable, in whichever manner. The Revantusks alone could represent Forest Trolls in the Horde pretty well, and Blizzard could always invent a new Forest Troll tribe.