Yeah, I brought this up a while ago when the game was still new.
Mixed species habitats are often put together like fish tanks. In a tropical fish tank, often you have "feature fish" and "community fish". Feature fish are the flashier ones, the bigger ones, the show-stealers. They draw the eyes, they're the primary point of the tank. Community fish are the more common, or mundane inhabitants, that live in bigger groups and 'flesh out' the tank.
It's the same with mixed species habitats in zoos. Usually you have a "feature species" (like the giraffes or elephants) and "community species" (the gazelles, zebra, and ostriches). Currently in Planet Zoo the lack of an African rhino is rather limiting. While some zoos do separate these animals into individual exhibits, even those that do still place a heavy focus on the 'feature species'. An example of this is my local, Orana Wildlife Park, which has no mixed-species exhibits but a lot of African herbivores. The zebras, giraffes, springbok, rhinos, waterbuck, and addax are all kept separately, but they do a daily giraffe feed and a rhino encounter because it's understood that these are the "Big Ones" that people want to see.
We do have the African elephant and the giraffe in PZ, but there are numerous "community species" that don't really generate the same appeal on their own. It would be nice to have an African rhino to be able to spread out the African savannah animals more evenly. Three big mixed exhibits works better than two (or for those of us who prefer to use the Indian elephant in zoos, an African rhino is almost completely necessary to flesh out the African section).
That said, I think one will be in the works. It's an animal in high-demand and for Frontier it wouldn't be as resource consuming as making brand new rigs such as for a kangaroo or something. The base rig is already there with the Indian rhino, so while it would still take work, it would likely be as much work as the polar bear rather than making a brand new animal.