The main group of ungulates I want to see more of are the small ungulates, as these tend to be fairly versatile in how they can be housed with many of them suitable for mixed enclosures and walkthrough displays. There are lots of species to choose from such as duikers, dik-diks, steenbok, pudu, muntjacs, chevrotains, Chinese water deer and klipspringers; any one of those would be appreciated greatly by me.
Special mention goes to the pygmy hog - it's the world's smallest wild pig which is kept in at least one public zoo (Assam State Zoo in India), has been kept and bred in at least a couple of European zoos in the past (London and Zurich, that I know of) and they are bred in large numbers at specialist breeding facilities for release into the wild. I think it is at one of these breeding facilities where this footage from Planet Earth was actually filmed. Although it's not a widespread species, they are definitely one of my favourite ungulates and would, I think, be a great addition to the game.
Source: https://www.youtube.com/watch?v=DOoWAZTgUWM