My choices for these groups include:
Pelecaniformes
The only essential for me is a pelican. I would prefer either the
great white pelican or
dalmatian pelican, as both would function well in many different geographical-themed areas of zoos, are both commonly-kept in open-topped and walkthrough enclosures and are some of the most charismatic birds that could reasonably fit into the game.
On the other end of the scale, the shoebill is near the top of the list of animals I don't want. As well as not being that common in captivity, they are almost impossible to breed (I believe they have only successfully bred in captivity on two occasions) and so almost all birds in zoos have come from the wild - it is a problem to the extent that the IUCN considers the capture of shoebill chicks for sale to zoos to be a threat to their wild survival.
Storks
The white stork is probably the most commonly-kept species and can live in large open-topped paddocks, including walkthrough enclosures and mixed displays with other animals such as hoofstock. Their cultural significance in Europe, and the fact that they represent Europe, Africa and West Asia are also important factors.
For me though, my preferred stork would be the
black stork - they too can be kept in large outdoor walkthrough enclosures and mixed displays, are very distinctive-looking birds and have a wider geographical and biome range - they breed from western Europe right across to the Asian Pacific coast, winter in India and Africa and also breed in southern Africa. They are kept in 160 zoos across Europe and Israel alone, so are certainly not an uncommon species.
This picture shows an open-topped walkthrough enclosure in the Netherlands, where both dalmatian pelicans and black storks are visible; they also live with cranes and three species of ungulate:
Home to Dalmatian Pelicans, Black Storks, Cranes, Blackbuck, Vietnamese Sika and Reeves' Muntjac. Definitely one of the highlights of an otherwise fairly generic park.
www.zoochat.com
Lower on my list is the
saddle-billed stork - a magnificent and beautifully-coloured bird that currently lives only in sub-Saharan Africa, although it did have great cultural importance to the ancient Egyptians. However, I have never seen one myself and only one zoo in Europe routinely breeds the species, so I cannot consider it essential.
The marabou is lower on my list - not only do I not find them particularly nice to look at, there are all sorts of problems associated with them in open paddocks in zoos. The major one is that, despite being often kept in mixed enclosures, they are not at all suitable for being housed in such a way. Many of the zoo associations do not recommend mixing them, as they rarely if ever breed in such situations and are at high risk of being killed by aggressive hoofstock. There is also a very distressing video on Youtube (which I won't link here) which shows a marabou in a Spanish zoo dragging a gazelle fawn into the enclosure moat and drowning it.
Iguania
A
chameleon is a must as an exhibit species - the veiled is the most commonly-kept species, while the panther is among the most colourful and the Mediterranean has a range covering three different continents. I would gladly accept any of those.
Probably the other agamid that verges on being essential in my mind is the
Asian water dragon. It would have a lot going for it, being the only Asian lizard in the game, attractive to look at, a species listed as Vulnerable in the wild and very common in captivity.
Gastropods
Probably no other gastropod is necessary, but to showcase the conservation of critically endangered snails that goes on in zoos, I would love to see the
Vietnamese giant magnolia snail Bertia cambojiensis included. It was thought to be extinct until they were rediscovered in 2012 and since then several zoos in the UK have taken them in and started breeding them, including London Zoo. It has the advantage over the similarly-threatened Partula and Madeira land snails by being very large in size.