For Completion, id say the Easiest is western/central Europe.
Red deer, Wild Boar, Red Fox, an aboreal Animal (red squirrel and marten would be good options, but even the racoon would cut it, even if nots from europe originally but still feature in many european focused collections as one of the few very common non native animals in our wildparks) and honestly, the european river otter would still be fun and a great pick for an eurasian fresh water animal.
America got the beaver so its just fair to get the otter for us.
The otter in general is very flexible, actually.
Just look at this bonkers Range spanning every single biome in the game.
For northern Europe/Skandinavia, only the River Otter, Wolverine and maybe Walrus feel like they are missing. Northern Asia in generall would also profit from a smaller deer, both roe deer and tufted deer fill that slot nicly, one having a greater range and the other being more special.
Japan is a funny one, as they only need the wild boar, red fox, sika deer and tanuki to feel complete, with the former 2 beimg very likly to be implemented just because of their massive ranges, but even so the tanuki still has a decent shot and if we muss out on one of them its allright if its the sika.
India is also very straight forward. Blackbuck and nilgai for ungulate representation, one or two monkeys, grey hulman and lion tailed Macaque being prefered but literally any monkey is welcome in my book, aswell as atleast one more bird and reptile.
The barheaded goose is a great candidate, but i would also be fine woth just a pelican with both obviously being prefered.
For the reptile i dont care which one, i just want more reptiles and im sure theres something in the ganges that would fit nicly into the game, for example a giant softshell turtle.
East Asia needs muntjaks and a bunch mire pheasents, nuff said.
Usa and Canada are fine imo, only standout requests would be my beloved trashpanda and a bird, where turkeys and maybe something like the brown pelican and sandhill crane come to mind.
An animal that i wouldnt say is needed,but would fill an open niche and be a nice sidegrade is the bobcat. Having a north american small cat that lives in deserts and woodlands, aswell as destinguishing itself with a reddish taint sounds fun and distinct enough to claim a spot in a clone pack.
I believe that Sub Rainforest africa actually is blue.
Think about it, most requested animals for africa are from the congo basin and above, but everything below has great representation.
While stuff like the honey badger, secretary bird and other big bird like the shorbill or marabu would be great, they arnt needed, making the area a nice perfect one allready.
Meanwhile i would down rank tropical cental africa into needs key animals to be complete, these being the red river hog and literally any monkey, but lezs take the de braza monkey and mantled guereza as our examples.
I would still appreciate a duiker in general, but thats mostly subjective and the game would totally be fine without a dwarf antelope.
Central Asia is quite the quick fix.
Domestic Yak, Takin and either Himalayan Thar and/or Markhor and we would be good.
But for fun, himalayan monal, bar headed goose, pallas cat and corsac fox all bring something unique to the table, the former being birds, the later being small carnivores, something the region is lacking and all 4 of them would need enclosures very distinct from every himalayan/central asian animal.
Saiga would also be dope.
Tropical South America needs a Coati and atleast 3 more monkeys. I also really like the agouti, coullared peccary and Brazillian tapir, because all three are grounded animals in a very aboreal and aquatic heavy region. But in all honesty, Tropical South America is the one region were the lack of aviary birds makes a noticeable difference, as they make up a big bulk of that area in most zoos.
Madagascar is cool, just Fossa and radiated tortoise are needed to make it feel complete imo, but more lemures never hurt.
Two non lemure animal i would really like to see are the malagasy giant rat and the white faced whistling duck.
The former is greatly endangerd with an expsnsive breeding program and looks like a kangaroo lite with rat features, while the other is a duck that ranges 2 continents and madagascar.
Its hard to be more objectivly a good addition then that.
Australia and Oceania as a whole need the most work imo, with the Emu and Bennets Wallaby being essentials.
Afterwards i want atleast 4 for more animals, so that we reach 12, where i dont really care what they are, but my favourites propaply would be the victoria crowned pidgeon, black swan, tassie devil and an aboreal monitor, but they could easily crank that up even more with more wallabys, a wombat, echidna, kiwi and honestly allmost anything that lives down there, i dont discriminate.
But baseline is, 6 more animals with emu and bennetts wallaby being a must.
New Zealand shouldnt be its own region imo. To small, only the kiwi, little blue penguin and fur seal come to mind and just in general its not a place present in international collections.
Would like a kuneku pig though.
Central America needs a coati, peccary, ocelot or margay and a monkey. Pipe dream also says manatee.
The other 3 regions just need animals in the furst place, each one of them could easily make alot out of even a scenery pack, but before theres a roster its hard to say whats needed to complete it without just mentioning most possibilitys.