I am not so pessimistic. I think, it is not to optimistic to have hope for another year of DLC, so at least 6 more DLC:
1. (North) America
2. Aviary pack
3. Eurasia
4. Desert and savannah
5. Tropical
6. Endangered/Fan Voting
This is almost exactly my thought too, though I wouldn't be entirely surprised if they end up combining Europe and North America into one pack - if so, my guess for the freed-up slot would be another biome-based pack (or split out desert and grassland), 'Islands', or some sort of nocturnal pack (possibly wishful thinking on my part - I just want night house stuff and an aye aye at some point!)
And yeah, an endangered species pack would basically just be a fan favourites pack by another name, which I'm all for!
EDIT: If there were to be more packs than this, I would think additional biomes would be the way to go, being a really easy way for the devs to add in popular 'missing' animals from previous packs, without going with 'Australia Pack 2' or whatever (which I think would be tricky from their point of view to not be seen as 'we messed up the first time, here's another go at it!'). Grassland could have maned wolf, emu, any number of African animals, Tropical could have another new world monkey (or two!), a gibbon, red river hog, and so on. If they did end up combining Europe and North America, I would think that a 'Temperate' pack would probably be primarily EU and NA animals.
(Sidenote - future biome packs is my tinfoil hat theory for the disconnect between theme and animals in the Africa Pack - they wanted to do the North Africa building theme, and the seemingly random assortment of African animals to accompany it were those they couldn't find room for in their vague future plans)