Assuming this is restricted to sub-Saharan Africa, I would say either 1) South Africa is more centralised and the Anglophones are more on top, and Cape Town develops as the sole capital, or 2) Zanzibar as a free city along the lines of Singapore.
Cape Town definitely has the ingredients to be a World City ranking with London, New York, Paris, and Berlin.
It has a mixed ethnic background, a strong historical background. It has a good climate. In the OTL it attracts lots of the rich and famous.
To make it work in the timescale I'd say let's imagine that after the Boer War rather than following a policy of reconciliation with the Boers, the British pursued a campaign of open immigration and non-racial property qualification for the vote. So maybe instead of Argentina being a magnet for millions of poor Italians, South Africa becomes so.
Perhaps it is permissible to see South Africa as a big multi-ethnic democracy with a very successful economy. IN those circumstances Cape Town could be the big world city.