Well, then sorry for being so tedious, but we have the right to state our wishes like everyone else.
If you are implying they are not even zoo animals you are wrong aswell. They are kept in normal zoos around the world, not only in marine parks. The only captive dolphins in Germany for example are kept in "normal" zoos - Zoo Duisburg (bottelnose dolphins and an Amazon river dolphin) and Zoo Nürnberg (bottlenose dolphins). Duisburg even had Belugas in the past. As far as I know all places who formerly kept dolphins in Germany were either zoos or amusement parks. Not marine parks.
Other examples are the Indianapolis Zoo and the Brookfield Zoo.
And then there is Loro Parque of course, which keeps bottlenose dolphins and orcas. Sure they may be the only "normal" zoo to keep orcas, but that still makes them an animal kept in zoos.
And at least theoretical, each zoo (in countries where its legal) could keep dolphins and orcas if they wanted and had the needed space (like in legal requirements), money and of course a source to get such animals legally in the first place. (Loro Parque got the orcas in 2006, thats not soo long ago, in my opinion)
So in that regard I think nothing speaks against adding dolphins/orcas/belugas to Planet Zoo.
Actually Ive seen people asking for animals who are much rarer in captivity than dolphins or even orcas are. For example Saigas. I would like them in the game too, no question. Currently they are only kept by two zoos, though. The last zoo who kept them in the west was the zoo from my home town, the Cologne Zoological Garden. They were pretty succesful with breeding them aswell, however one evening in 2006 the keepers were unable to get the females and their young inside their stable for the night, so they left them outside. During this night however a wild fox was able to break into their enclosure, trying to prey on their young (I dont know if it succeded). This set the whole herd of in panic mode. Saigas are very shy, nervous with a very high flight instinct. So they tried to flee... Run into the fences an anything. Well, the next morning when the keepers found them, those who survived the night had to be put down, because they were all injured so badly. The zoo basically lost the whole herd, only the 2 males survived, as they were kept seperated from the females. They kept Saigas until the last male died, I think that was either in 2009 or 2010. But their breeding efforts ended that night.
And I as far as I know the keeping of Saigas in other zoos quite often ended in an alike fashion. They might not look like it, but they are not doing that well in a restricted space.
But back to the topic about cetaceans, sure they are controversial, but I have lots of faith that the Planet Zoo team would be able to add them in an educational way that would actually make them fit with their message. In my opinion the could just ignore the controversy surrounding them. But if they wanted to touch it, then I am sure they would do it well
About the technical part, I dont know anything about it, but from what is already possible regarding the water in PZ, marine animals should be possible. Hippos can already go under water at least. I know its not real swimming/diving they are doing, but they still can do it. Sure, there are animations missing and there would need to be some special entrances and probaly some other things I cant think of right now - but I do think the basics for marine mammals are already in the game.
Anyways, I dont think its already decided whatever they will add orcas in the first place or not. But if people actually tell Frontier they want them, they may be more likley to be added.
However, when I say whales I dont mean anything bigger than an orca (who are actually dolphins, but dolphins are also whales, so

) I do think truebig whales are out of question. I know we will be very lucky if we even get orcas in the first place, maybe also belugas, pilot whales and such - but I dont think we would ever get something like sperm whales or even blue whales. I wouldnt be mad if they added them, but they are too big and not even kept captivity in real life, so I dont see them happening.
I am still all for Frontier making an cetacean DLC only, so that players who wouldnt want them could choose not to get them in the first place. But still get seals, otters, manatees and such in another pack if thats what they wanted.
Its still too early to say they will never add dolphins/whales to the game. Of course I hope to be on the "winner side". I could get dissapointed, sure, but as of now there is still some hope.
But enough of that, I already wrote more about it than I wanted
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Aviaries would be cool, but I sure hope they just wouldnt be exihibits. Well, at least not for bigger birds.
Hmm, for petting zoos I would like that aswell, but if that would really include the guest petting the animals I think that would be harder to implent considering how the guests work/move in the game. Maybe I am totally wrong, but I just think its much harder to change things about the guests than adding new animals
I would also like monkeys, they would be really cool
An Australian DLC, aswell as an European one would also be good options in my opinion.
I only hope they will also release bigger DLCs from time to time, even if they would cost full price.