Make the search on Matrix simpler while you are exploring routes. It's a betting certainty that you will fly via JNB. And on Matrix if you put in CMH/JNB a whole host of connections via Europe are shown - as well as some via the Middle East. You could also experiment with putting in a bigger hub in the USA as your starting point so that more routings are shown. ORD/JNB, for example, shows double the number of routings as CMH/JNB.

Another method is to use the "Nearby" option on Matrix - e.g. look at all routings that start within 1000 miles of CMH. But it doesn't seem to throw up as many options as using major hubs like ORD or NYC.

Once you have an idea of what transit point(s) excite you, you can put the complete routing into Matrix using the multi-city tab.