I feel like aviaries will end the game. Meaning it will be the last DLC Frontier puts out. Either that or an endangered species pack, which would be nice as well. With an aviary pack unlikely for a December 2021 DLC, and fact that there is 4 more DLCs left (after this one), I don't think it would make sense for the next DLC (after this one) to be aviary and then "come back down to earth" for the remaining ones. But, who knows, after this probable Europe DLC, they would have covered pretty much all geographical regions of earth (except Antarctica, but ya know). So, the direction is pretty unclear.
Anyway, I'm hoping for an update announcement tomorrow, but we may have to wait until next week for that unfortunately. If we do get it tomorrow, then we potentially could see a DLC announcement on Thursday and a release on the 7th, which is unlikely but definitely possible.
Both dlc suggestions would be bad "last" packs for the game, from every single perspective, be it gameplay, bugfixes, buissness, a possible sequel and ofcours ours, the consumers.
Aviaries would be bad as the last pack, because its really not cost effective to develop a whole new system for 1 10 bucks pack. Also, flying screams for bugs, bugs that wont get fixed if the lifecycle of the game will be over afterwards.
For all kinds of fanfavorite packs the answer is simple. Aslong as we can agree on animals to put in them, they could be a dlc for each animal, that they headline. More animals and props for us, more money for frontier, win win for everybody.
And before you start "oh but thats why its the last pack, there wont be anything afterwards" no its not. From what we have seen from frontier, they like to make sequels, even if they are apparantly allready milked for dlcs like planco or jwe. And what will these games get? Dlc of course. So even if some fanfavorites wont make it in planet zoo, im sure they will come in a sequel, as it would be very weird for frontier to sequel everything, but their best performing game.
My take is, that the "last" dlc should just be a normal dlc, maybe something that caps of the theme of what ever dlc cycle we have, like europe if support ends now as the last continent or the last biome if biome packs will be the next big thing