In an ideal world it would have all the animals from planet zoo 1 plus more but that is very unrealistic because it will make the game really expensive to both frontier and the players it will also make dlc even harder to make.Going off of this ..
I don't think it would make business sense to have a Planet Zoo 2 with all existing animals of Planet Zoo 1 thrown in as a base or initial DLC version without the hefty price tag.
However, if this possible Planet Zoo 2 had plenty that is new - say, flying birds - small aquariums, the ability to have multiple staff doors on habitats for those HUGE habitats we love to build but have problems with feeding and cleaning because of the one entry gate limitation improved graphics/animations etc and I am sure many other things that Planet Zoo 1 doesn't have, they could include the favourite mammals and insects from Planet Zoo 1 in the base game and then include the others over time in DLCs similar to PZ1.
Or thinking outside the box .. have Planet Zoo 1 items be compatible with Planet Zoo 2 so that the old DLCs will still have relevance in the new game. All dependant on how PZ2 would work.
Besides that another really good but I have no idea how feasible scenario is animal transfers so if you have planet zoo 1 and any of its dlc those animals are then compatible with planet zoo 2. This would keep old players and may even bring traffic to the old game but I have no idea if steam is capable of this.
This combined with an almost remake style where they essentially re release planet zoo 1 with a bunch of improvements, new mechanics and new animals to the base game alongside the same dlc would be the ideal scenario.
Existing players get to keep the dlc animals they payed for and are essentially just paying an upgrade price new players will have a multitude of dlc to buy straight away if they choose and frontier could continue to make more dlc focusing on animals that fit whatever new mechanics they added.
I know this scenario is almost impossible but its what I want because I dont want to lose any of the animals between games.