3.55 would be way too late to add the Nidhogg bird to the vendor.
I'd expect them to be adding the Sophia and Zurvan birds to the totem exchange vendor in either 3.55 or 3.56 (2.x primal mounts droprates were buffed in 2.56), as a further incentive for players to continue playing the game. You have to keep in mind that the entirety of the 3.5 patch (3.55 included) will be around for longer than the other patches. Going by the current patch cycle, we'll likely see 3.5 mid or late january, followed by a space of almost 5 months until the expansion's early summer release.