I'm always torn on this subject.
On the one hand, the southern white rhinoceros is the species I most commonly see in zoos. As such it as an emotional value to me so I'd really love to see it in-game. It is also the only species that lives in herds (black rhinoceroses do group up, but usually only on occasion).
On the other hand, the eastern black rhinoceros is critically endangered and so it has conservation value.
If I had to choose just one out of those two I'd be stuck. Either way, the rhinoceros is the obvious missing piece from big African exhibits so it's definitely a must-have at some point. If we had a pack with one of those in it, the meerkat, a baboon, and, say, the southern ground hornbill I'd be thrilled (all very savannah-centric, of course, so I wouldn't expect this limited selection for a pack designed to cover all of Africa, but a man can dream).
As for the Asian rhinos, I would love to see the Sumatran (after one or both of the African species). The game already has a handful of animals that aren't generally kept in captivity (or are very rarely), so I don't think that's a pre-requesite for inclusion, and in any case Cincinnati Zoo successfully bred the Sumatran rhino in captivity once. They've stopped that captive programme now due to the inviability of it but for a while it seemed like it was going to work. Recreating that and making it successful in-game would be a lot of fun.
You could make a similar argument for the Javan rhinoceros (although it has never been successfuly kept in captivity), but as others have pointed out in terms of looks it isn't enormously different from the Indian.