I'm not entirely convinced we're done with regional packs. Most of Asia hasn't been covered yet...
But for biomes, my best guesses would be:
Islands:
Tasmanian Devil
Kiwi
Japanese Serrow
Falklands Steamer Duck
ex. Lorde Howe Stick Insect
Various scenery pieces and plants
Drylands (because wetlands - dryland has a nice symmetry)
- Prezwalski's horse
- Saiga (gotta have a snufflelope)
- Emu
- Yellowfooted rock wallaby
- Hamadryas baboon
- Stripped hyena
- Scimitar horned addax
- Chacoan peccary
ex. frilled lizard
Highlands (let's stick with the blank-lands theme)
- A bear (either spectacled or American black bear)
- Pallas cat
- Himalayan monal
- Takin
ex. ?
But switch the bear for a leopard and this would also be a sweet central Asian booster pack
Canopy
- Fossa
- Golden Tamarin
- Mantled Guereza
- Tree kangaroo
- Racoon
- Grey crowned crane (the only perching crane)
- Lar gibbon
- Indian giant squirrel
- Sloth
- Tamandua
- Ocelot
- Coati
- Leopards go in trees sometimes
- I don't think we'll get all of these in one pack, but I think there are a lot of animals that could go in a climbing based pack
ex. any Chameleon
Equatorial (I'm stretching here, because there are favorites that still need to be squeezed in + let's add some more birds)
- Maned Wolf
- Red River Hog
- Secretary Bird
- Victoria crowned pigeon
ex. emerald tree boa