I honestly don't believe we're getting ZT2 style expansion. Expansions took much longer to make and even when we compare ZT2 with all expansions ( except EA, reasonably ) with PZ in current state, counting latest DLC-The PZ beats it in number of animals:
I've not taken into account exhibit animals from PZ and extinct animals from ZT2 since they do not fit into the other game.
forums.frontier.co.uk
So it simply and sadly isn't worth the time. This system is way quicker, generates more income and isn't lacking behind in quantity of content. Now quality would be subjective but even ZT2 didn't have aviary birds.
So if we're getting birds and aviary at all, I think we should manage our expectations and compromise what we can. Still I don't think that 8 birds plus possible later additions is a bad deal.
But I totally understand that everyone has their passions and, you perhaps, waited just for the birds from the start, and I can see how 8-10 would feel underwhelming for you. But we don't know anything yet, not even if there is one aviary pack let alone the content of it.
I do know however that they listen to community and that they're ready to meet the needs of their fans. So I trust we'll get a solution that will leave both sides satisfied
On top of that we do not know that this year is the last one - DLC wise, only thing we know for sure that we are getting four DLCs next year, and not much else.
I see plenty of room for optimism there