Planet Zoo base roster had around 80 animals, and to be honest it had alot of key animals missing and had a lot of holes... and now with all the DLC's and everything the animal roster up to 210. There are still alot of important animals missing but its much much more complete and has alot of key animals.
Flying birds and Aquatic animals are the two most wanted animals in planet zoo, so I would assume they are probably going to be adding them in Planet Zoo 2, 100% at least one of them but im fairly certain they will be going for both as it would be a big selling point.
Considering they would at least need to add 15-20 flying and 10-15 aquatic animals to have some sort of decent representation that makes 25-35 animals just from that, so they can't possibly have a roster of only 80 animals like before, we would have an even more lacking roster than Planet Zoo (of non flying and aquatic animals)... surely they realize that would be suicide for them, going from 210 animals to like 40 lol.
I would hope they are smart about this, take their time and give us at least around 100 (less than half of what we have in PZ1 now) animals at the very least (not including the new birds and aquatic animals).