Rainforest: Almost certainly. Gives us gibbons, sloths, more monkeys. All things we desperately need.
Desert: Almost certainly. Fits the "summer" dlc quite well, and we still have few desert animals. Coatimundi, addax, hamadryas baboon etc.
Grasslands. Almost certainly. Just a really easy theme to make, and so many animals fit into it. Wombat, tassie devil, maned wolf, olive baboon etc.
Islands: Highly probable. A fun theme that people often ask for. Helps for any Oceania animals as well as the fossa from Madagascar.
Taiga: Highly probable. There are still some cold weather animals we're missing out on, and alpine animals we're missing out on.
Temperate forest and tundra: Highly doubtful. Tundra is too niche and temperate forest has gotten a lot of animals between NA and EU packs and has good representation.
The last one I could maybe see is coastlines. I would love it, but won't hold out hope.