I think with ungulates now the main thing we need is just filler stuff, especially some small guys. Dik-Dik, Muntjac, Mouse-Deer, Pudu, and Duiker would all be amazing and add so much... Give me all of them lol.
Also I didn't really realise until recently but Europe is so lucky to have two deer (even I do wish we had roe too) that I really think getting some for Asia or NA would be super helpful: Sika Deer, Chital, or Pere David's Deer would all be excellent and are different to each other in niche. And for North America, the White-Tailed Deer would be a nice filler ungulate - Also the Wapiti works for both!!
Some other things I'd like to see would be: Some more for South America, I think getting a wild camelid would go a long way and honestly a second peccary too, they all look pretty different. A second giraffe or zebra for variety sake... And hot take for me but I really want the impala, for me it is just too iconic but I know thats not a popular opinion. Musk Ox and Aoudad would round out the caprines perfectly filling new niches and I do think the Chamois would be lovely if we could mix it with Alpine Ibex.
Am I missing anything? I do love an oddball, I'd be very happy to see the vampiric looking Siberian Musk Deer. Oh and of course the Blackbuck, seriously Frontier I think this might be the most robbed animal yet.