There's a few additional domestics I would get behind (e.g., Shetland pony, pygmy goat, chinchilla) but certainly not as part of a final pack. Chinchillas, in particular, would be useful because the habitats built for them could vary and because they would inevitably get a dust bath enrichment.
Unpopular take, though: While smaller mammals like guinea pigs, rats, mice, chinchillas, and so on of all sorts could work fine in the present game format they would work even better as part of a sequel with better hit boxes and a new, more customizable exhibit format. But this is also just me having an innate desire to build a Small Mammal/Nocturnal/Mouse House similar to what they have at the Bronx Zoo, London Zoo, National Zoo, etc. Haha.