Here are a few of my favourites:
Eurasian red squirrel - here in the UK at least, these are fairly common zoo animals and can be kept in large open-topped walkthrough enclosures. From what I gather, they are perhaps not as common in 'mainstream' zoos on the continent, perhaps because they have not seen the catastrophic declines there that they have in Britain.
Eurasian oystercatcher - I have seen these kept in open-topped enclosures at bird parks and zoos. They are rather pretty and charismatic birds that I'd love to see in-game, but equally they are a species I could drive five minutes to the coast and see in the wild.
Bar-headed goose - While they are an exotic species, they are very common almost to the point of being mundane. I have seen them not only in zoos, but at ornamental gardens, in the grounds of castles and on one occasion at a garden centre. I have also seen a pair of feral bar-heads with goslings on a nature reserve. One of my most-wanted species in the game.
Eurasian otter - I have seen them in the wild a few times, but they would add so much to the game that I'd love to see them. It would be good to get a solitary species of otter, which could reasonably be said to be able to occupy every in-game biome.
Not sure if it really falls into this category, but the grivet monkey is such a vanilla species - not especially large, colourful or impressive in appearance - but still would have tons of utility for the game, would add a much-needed smaller African monkey and is one of the most common monkeys in captivity. They are probably my most-wanted primate.
