These are all of my choices from these four different groups:
Small Cats
I do think at least one or two true small cats would really add to the game. The Asiatic golden cat is probably my favourite species, but their rarity in captivity means they can hardly be considered essential. The black-footed cat is still found in a number of American zoos but is now absent in Europe - they are one of the smallest of all wild cats and should have correspondingly tiny habitat requirements, which would make for an interesting animal. And European wildcats are very commonly-kept in zoos in Europe and would add to a temperate forest or Mediterranean display. I think my two favoured choices however are the margay (the most arboreal of all cats, very different from anything else currently in-game) from Central and South America and the fishing cat (a semi-aquatic species) from South and Southeast Asia. If just one slot were available, I would probably err towards the fishing cat.
Xenarthrans
A Linne's two-toed sloth would be nice, but the ongoing discussions on how it would actually work in-game makes me a bit cautious to say it is necessary. The last thing I would want is for the animal to come to the game and then feel unusable because it behaves so unrealistically. A Southern tamandua would also be a good addition, but I wouldn't say they are an essential animal.
Geckos
As one of the biggest lizard families, I would say at least two species of gecko are necessary - one arboreal and one terrestrial species. While the leopard gecko is the most commonly-kept terrestrial gecko, I would go for a more impressive but otherwise similar species - the Iranian fat-tailed gecko Eublepharis angramainyu (also known as the Iraqi eyelid gecko). It is in the same genus as the leopard gecko, but has the advantage of being considerably bigger. The picture below shows an Iranian fat-tailed and leopard gecko side-by-side; the leopard gecko is the lower animal. They are not hugely common in zoos - two zoos listed on Zootierliste, plus a slightly larger number of zoos in America, but their impressive size ultimately wins the day for me.
As for an arboreal gecko, the most obvious picks are either the tokay or Madagascar giant day gecko. However, I will vouch this time for the turquoise dwarf gecko. It is a fairly small species from montane rainforests in Tanzania, but they make up for their diminutive size by being brightly coloured, critically endangered and common in zoos - here in the UK, they are more common than tokay geckos and draw with the Madagascar giant day gecko, both being kept in 20 zoos.
Insects
This is quite a difficult one for me, personally speaking. I find insects to be one of the most important animals in a zoo (I will genuinely spend more time looking at insects than, say, elephants or meerkats) and currently there are surprisingly few good options - I would say only two of the beetles, the cockroach, leaf insect and two of the five butterflies are regular in captivity, with all the others being either very localised (the three temperate butterflies) or absent (titan beetle). I have divided them up into several major taxonomic groups:
Beetles - There are so many options for beetles that it is quite insane, and having to choose the most important one verges on impossible. Perhaps the most commonly-kept are the Pachnoda sun beetles, but they are often effective as exhibits in huge numbers as they are not very large. A slightly larger African species that could work well in smaller numbers are the Eudicella buffalo beetles; they are all brightly coloured, the males have impressive horns on their heads and they are all kept and bred commonly in captivity. Below is the jade-headed buffalo beetle, Eudicella smithii.
Cockroaches - The Madagascar hissing cockroach is the most obvious and commonly-kept species, but I do have a real soft spot for the Simandoa cave cockroach. Quite colourful, highly sociable and currently thought to be extinct in the wild - their entire native range (a single cave in Guinea) was destroyed to build a bauxite mine in 2004. They share the cave with Egyptian fruit bats, which would link them very well to the current walkthrough exhibit species.
Stick insects - While we have the leaf insect, a more standard stick insect is still missing. I would say at least two species are needed - even my local zoo, which only has five invertebrate species, keeps two species. One spot I would reserve for the golden-eyed stick insect Peruphasma schultei, a critically endangered species from the forests of Peru. The second spot would be for a more traditional species - the giant stick insect Tirachoidea jiangfenglingensis of Southeast Asia is one of the largest stick insects, making it my preferred choice. The below pictures shown the golden-eyed on the left and the giant on the right.
Grasshoppers - Probably the most common grasshopper in captivity is the desert locust, but I would prefer something more interesting. A colourful smaller grasshopper, either the Surinam grasshopper Tetrataenia surinama of South America or the painted grasshopper Zonocerus variegatus of tropical Africa would be my favourite choice. The Surinam grasshopper is in the right-hand picture below, with the painted grasshopper in the left-hand one.
Mantises - One of the biggest missing species for me is a praying mantis. What I think is necessary is a striking-looking mantis, which is large enough to warrant its big enclosure and, most importantly, not so cannibalistic that only one or two could be kept in an exhibit. Fortunately, there is one that fits all three requirements perfectly - the giant devil's flower mantis Idolomantis diabolica of the African savannah. They are among the world's largest mantises and because they only eat small flying insects they can be kept communally.
True bugs - While most of this family are not kept in captivity, I would venture towards saying that an assassin bug is necessary. The white-spotted assassin bug is the most commonly-kept species, but I would err towards the horrid king assassin bug Psytalla horrida. They are also commonly kept and bred in captivity, but have the added advantage of being the world's largest assassin bug species.
Butterflies - I do wish that the butterflies had been delayed to a tropical pack, so more standard butterfly house species could be included. With the chances of lots more butterflies unlikely, I have chosen what I consider to be the single most important species - the paper kite butterfly Idea leuconoe.
Social insects - I'm not certain how well social insects would work, but I do think that honeybees are an important addition, plus a species of ant. While leaf-cutting ants are the most famous, they are actually surprisingly difficult to keep. I might err towards a more arid-adapted species such as the honeypot ant Myrmecocystus mexicanus from the deserts of North America - the hanging repletes would be easier to animate, with the workers just running around rapidly on the surface.
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