Have more European immigrants into the Dutch Colony for one. Throughout the entire period of Dutch rule, only 2,044 immigrants arrived in the Cape. Due to a salubrious climate and high birth rate they multiplied. However, few of the immigrants arrived after 1700. Few Dutch settled in their colonies, however the Dutch did not seem opposed to settling foreigners in the colonies in New Netherlands or South Africa.
The Netherlands had quite a few English Puritans and they were in the Netherlands and they may have been settled in South Africa. In 1645-46 Puritans attempted to establish a colony in Madagascar, so they would not be opposed to settling in Africa. A small group of these, say 1,000 would create around 500,000 descendants by 1900.
Many of the immigrants were actually from Germany, Scandinavia and French Huguenots. Many Germans from the Palitanate region of Germany settled in the Netherlands, especially Rotterdam during the War of Spanish Succession. Some 2,100 Palatinate Germans were settled in New York in 1711. The Netherlands could have easily settled a similar number in South Africa. By 1900 their descendants would add another 300,000 Europeans to South Africa (taking to account the similar birth rate).
Take in an additional 20,000 Europeans during the 1700-1750 period and you have 1.5 million whites added to the population by 1900. Another 10,000 immigrants in the 1750-1770 period would add another 400,000 Europeans to the population in 1900. So by 1900 there would be 3.5 million "Afrikaners" in South Africa vs 850,000. The black and coloured population would be around 3.7 million by comparison, today they would number over 11 million.