The MOST? Aviary expansion. Not just an aviary pack, but a full aviary expansion DLC. We need so many birds, there's no way a simple DLC pack will satisfy me!
After that, North American DLC. I'm torn on this one, though. We are missing a lot of iconic animals from NA, more than we could fit in a standard pack, but we are ALSO missing any kind of NA native artistic perspective in the game. We absolutely need a build set from one of the indigenous traditions up here! It is a major gap in our architecture and art assets right now.
So I guess I'm really hoping for a standard NA Pack DLC with a build set, then maybe we could fit some of the other animals in other packs. (Coyotes for instance could go in a canine pack down the road.)
As for which DLCs would I be willing to buy? All of them, lol. All of them forever.