That's certainly a question.........
I guess one way to go about it is the long and tedious route - zebra and Thomson's gamelle, zebra and gemsbok, zebra and impala .... not bad if you have a great deal of willpower and free time.
Though the issue of temprament is always an interesting one. In the same manner that a full-grown shrew is many times larger than an embryonic fin whale, many mixes rely on the temprament of the animals involved. I recall an Italian safari park had a young hippopotamus mixed with other African fauna - something that would be much more difficult to accomplish with a full-grown hippo. And sometimes architecture helps as well - some safari parks, should they choose to include hippos at their drive through, make sure to have a large lake or something of sort. The hippos stay here most of the day, and so remain rather docile. I imagine also that this would be possible with elephant cows, but good elephant husbandry requires that should cows be able to conceive a calf, then they should be able to.
And some mixes do stagnate over time. A safari park in the United Kingdom had little problem with its Congo Buffalo for a while ... until one day when one decided to attack and kill an antelope. And so it is that this is a question with no real set of answers.