Although it isn't one of my favorite animals, I still like the musk ox. It might make a good anniversary animal. I'm not really sure how it would fit into a pack, and it is different from the DLC stuff. It's 16th on the wishlist, so it should probably be added at some point.
Sorry, wrong thread about anniversary.
As for headliners for animal packs, I would stick with what is popular on the wishlist at the time. Big carnivores like the maned wolf and bears, small cats, monkeys, colorful birds, and smaller animals like the crested porcupine and sloth could gain popularity.
I like small cats as headliners because they generate many internet memes and popular youtube channels. I think they have the best chance of bringing in new people.