Based on what I read here and elsewhere, I'm proposing an alternate history for Dutch South Africa:
In 1806, the Dutch defeat the British near Cape Town, and the Cape Colony is kept under Dutch control as a result. The Cape Colony keeps on expanding slowly, and undergoes numerous wars with the native tribes like the Xhosa. At the same time, the Cape Colony attracts migrants first mainly from the Netherlands, and then also from Germany, Scandinavia, and elsewhere. Meanwhile, the Trekboers would start making their migrations on a small scale for the time being, given their distrust towards authority. These would ultimately bring them to the Orange Free State and the Transvaal.
Either one of two things could happen in terms of territorial organization: The Cape Colony successfully rebels against the Netherlands sometime in the mid-19th century (cf. the American Revolution), or the Cape Colony eventually gets dominion-like status from the Netherlands (cf. Canada or Australia). In the first possibility, the OFS and Transvaal will go along with the Cape Colony, while in the second possibility, the Trekboer areas in the OFS and Transvaal just might split off from the Cape Colony in rebellion. During all this time, the British establish a colony in Natal (and OTL Durban is known here as Port Natal), along with keeping the area of Simon's Town.
When diamonds are discovered in and around Kimberley, the Cape Colony and any of the possible Boer republics claim the diamond areas, which ultimately are awarded to the Cape Colony. The Witwatersrand gold goes to the Transvaal (or to the Cape Colony, depending on the scenario). These discoveries are made 20-30 years later than OTL because of the delay in Trekker migrations. Both of these discoveries attract many migrants from the Netherlands and elsewhere.
The Portuguese, meanwhile, claim the area between present-day Angola and Mozambique - and hence colonize OTL Rhodesia and possibly also Bechuanaland, while the Germans colonize Namibia until World War I as OTL.
If the Cape Colony itself was awarded a dominion-like status, and the Boer republics split off, maybe all these would eventually federate into one country - and Natal would totally be its own country.
Today, the country resulting from the Cape Colony is more or less developed, though with lingering racism issues (never quite as bad as OTL apartheid) and poverty among the non-white majority. Same thing with Natal. Trying to think, though, whether Johannesburg still overtakes Cape Town as the largest city in the country. I think it should.
How does all this sound?