Objectively speaking, at this point, with the exception of primates, the rest are difficult.
- - The birds will be saved for an aviary expansion in Planet Zoo 2
- - Most rodents are mouse-sized (and at most, they'd be terrarium animals, which aren't made anymore)
- - and generally, I doubt they will put more than one reptile in future DLCs
Counter argument:
1. Plenty of birds that are hábitat material and popular: ducks, pelicans, storks, pheasants, spoonbill, ibises, secretary, shoebill, grey crowned crane, etc. There is no reason why Frontier couldn't add these types to the game right now other than that they don't like them / don't want to.
2. We have prairie dogs, and most of the rodents currently requested are largesr than prairie dogs: Patagonian mara, agoutis, Alpine marmot, coendu, North american porcupine, Luzón giant cloud rat, even giant squirrel.
3. For hábitat reptiles is more limited but we are still missing a monitor for Australia, the argentina tegu is popular option for a non monitor reptiles (since iguanas belong in boxes in this game unfortunally) and we have been asking for an african crocodile and larger fresh water turtles for a long time as well. So no need of more than one per DLC if they include one in every DLC.