I decided to recall when the other Raiding with Leashes achievements were added:
Raiding with Leashes (Classic): patch 5.1
Raiding with Leashes II: Attunement Edition (TBC I): patch 5.3
Raiding with Leashes III: Drinkin' From the Sunwell (TBC II): patch 6.1
According to this, it seemed that 6.3 would be the next one, but probably there will be no 6.3,
so who knows, maybe 6.2.3 is that one?
Talking about Legion, I need to think a bit longer about that. ;P
We can guess so far that it will include first WotLK raids up to Ulduar.
(Deleted a list, as it was confusing the readers.)
I'd deny Vault of Archavon, as it's PvP-conditional. Naxxramas was a remake, included in its former tier and The Eye of Eternity is too compact.
Saying this, the main focus is Ulduar and maybe The Obsidian Sanctum.
I've even given already a name to the achievement: A World Worth Taming!
Well, but what could be included here?
I was told that some Ulduar-themed companions were already created as the part of promotion.
And a next lashling isn't something what could be my priority, even if there are only two B/B right now ingame.