You're either have literally know understanding of ANY of the lore in this game or are one masterful troll. You claim it makes no sense for the Dark Iron to be with the alliance because they're "old enemies" but you don't seem to understand that the Dark Iron dwarf's Queen is literally part of the Council of the Three Hammers that is literally part of the Alliance now. The Dark Iron becoming playable is just a formality at this point and soon I'm sure they'll do the same with the Wildhammer. Also going back to the old enemies comment, you clearly don't get that the Zandalari trolls absolutely hate the Dark Spear Trolls which are the tribe that is currently affiliated with the Horde, so just because they're old enemies doesn't mean anything. The quest chains will explain why.
Most likely the quest chain for the Dark Iron will be helping Queen-Regent Moira deal with the Dark Iron civil war that has been going on since the death of Emperor Dagran Thaurissan (her late husband). There are still a ton of Dark Irons who don't like the fact that she allied with the Bronzebeard and Wildhammer dwarfs. Honestly it makes MORE sense for the Dark Iron to become playable than the Zandalari. As Horde we're going to have to do something pretty damn major in order to get the Zandalari to play nice with the Dark Spear.