Every type of biome has a "must have" type of animal for me. Grassland's is Maned Wolf, even though it's not even my most wanted from the lineup below. That's probably because I don't see another biome theme it could fit in, whereas I could see others that I might want more than the Maned Wolf.
I'd add in one of the Wombats for sure. They're my top want right now, but I don't consider them the "must have" for this biome since they can appear in others as well. Though it makes the most sense to put them here.
Masai Giraffe - I know not a popular choice, and is in fact my hope for the anniversary animal if we get one. But this pack would likely have an animal that's mostly a clone/reskin and this fits. Giraffes are absolutely an iconic zoo animal, and we only have one to choose from. It would be good to have a second option.
Secretary Bird - We'd hope for a bird and this is a popular choice. An emu would also be an option but they could also fit in the desert biome, I believe.
Patagonian Mara - while not unpopular, it would also serve as good wild card pick.
Saiga Antelope - I know, me suggesting an antelope! But I do actually like these. They are very visibly unique and would give us an Asian antelope, which we don't have a lot of right now. Also a "wild card" type of pick, while at the same time not being unpopular.
Baboon - a primate, not sure on what species actually, but overall baboons are very much requested in general.