Example 1: We shouldn't have Saiga because they aren't any in captivity.
Ignoring the fact that Saigas are kept in captivity, I'm personally very lenient about captive presence, as I see anything that's ever been held in captivity as an option; but I always had a hard time with the fact that people struggle with the idea that some people prefer to have animals that are kept in a captivity.
But to me that just makes perfect sense? Like, if someone were to make a pizzeria simulator game, you'd be expecting them to add pizzas as a food to serve right? You'd also find it weird if part of the gameplay would be for you to make hamburgers and fries?
The same goes for a zoo simulator game for me tbh. It's core is to simulate a zoo, so you'd want animals that are kept in zoos. Like that doesn't seem rocket science to me. That doesn't mean that I don't get that people want all kinds of animals either; but it has always been so strange to me that that sentiment doesn't seem to go both ways.
If we had Marine Mania and Extinct Animals for Zoo Tycoon 2, why wouldn't similar packs be something Frontier could potentially do for Planet Zoo?
Because the context is different.
For extinct animals, it's just a no go completely. Even your "focusing on modern extinct animals or only prehistoric mammals" isn't going to fly, I can guarantee you that Universal left no gap for Frontier to add extinct animals of any kind to any zoo game of their own making for the foreseeable future, because that would mean that they couldn't add it to their own games and they're not going to cut that possibility. I know that's a hard thing to hear for some people, but even a recently extinct species DLC is far of into the unlikely territory because of JWE. Even if the movie franchise itself doesn't even remotely touch it
It might feel like a shame, but that's how the industry works, and it's logical to do so. Otherwise they could cash on making it for Universal and then cash again by making their own game which is better. It's just not going to happen. These kind of contracts are really strict, I remember that I couldn't touch any kind of medical related project when I changed jobs from my previous job, whilst I was only working in a very very specific subset of healthcare.
Then why no Marine Mania? Simple as well really, the world is too polarized atm. And this is coming from someone who wants those animals in the game as well and believes that with proper research we're at least capable of keeping a bunch of these animals well in captivity. But it's controversial and whilst this wasn't the case when Marine Mania was released; nowadays social media becomes a cesspool in an instant. Frontier not touching it isn't most likely isn't necessarily because they don't agree with it on an ethical level; it's more to avoid the very vocal backlash that people who are against it will bring. And at the end of the day, I'd much rather not have a bunch of CMs and social media managers deal with endless slop and harassment if that means that I'm not getting a dolphin in my game. It's a game after all, the well being of the people who develop the game is more important to me.
So I don't think the majority of the people out there are against an extinct animals pack or a marine animals pack. Yes there are people who don't want it, but heck, there were
plenty of people who didn't when ZT1 and ZT2 was a thing. To an extant that there are people who didn't have the updated zookeeper AI for years because they never bought Extinct Animals. But I still don't think that's it.
I think it's more that people have come to understand why these packs are really unlikely to happen and moved on really.
I'd rather Frontier make a new game "Planet Aquarium"
Here's my hot take: any attempt at a Planet Aquarium is doomed to fail.
There's basically three niches in today's aquarium simulator market. You've got the simple "I want to see digital fish swimming on my screen" niche, the "I want to build zoos with fish" niche or the "I want to simulate keeping fish in aquariums as realistic as possible with lots of management" niche.
The first niche is quite filled, and so is the third one. Especially the third one, because a lot of aquarium games seem to have pretty in depth management. And let's call a spade a spade here, that's just not Frontier's strongest asset. They've got the creative building part down and they're incredibly good at it, but the common critique any Planet game seems to get is that the management side is really shallow. Could Frontier try to compete in that market? Of course! Is it smart to do so in their current financial situation? I'd personally say not at all.
Frontiers best shot is to integrate it into their zoo franchise, because frankly that's the niche that has the least amount of competition in the market. And as you said, that won't happen as a DLC, if that's going to happen that's most likely going to happen in a sequel. They wouldn't even have to go far, with a bunch of little tweaks the current exhibit system works wonders and scratches the itch of the niche described earlier enough.
But an entire game about aquariums from Frontier? Nah. I don't see it happen. The people who are heavy in to management will stay with the options they have right now and the people who wanted fish integrated in Planet Zoo would be disappointed. Releasing a sequel where fish are on of the big hooks (pardon the pun) like waterparks were with PC2 is a much, much safer bet.