The classic routes from CT northwards are basically along the west coast to Namibia or the easterly parts of Southern Africa either towards Mozambique or Zimbabwe. Both have their special landscapes and scenic highlights. Here are some ideas and tips to plan your trip.
On the NAM route don't miss the Cederberg Mountains. A great trip is along the eastern gravel road to Wupperthal, then across the Pakhuis Pass to Clanwilliam and then north along the N7. About 100 kms after the NAM border (Noordoewer) turn west to visit the famous Fish River Canyon. From there you could vote for the quaint town of Luederitz, the first (European) settlement in NAM and then to Sesriem/Sossousvlei to have a look at the grand red sanddunes. Swakopmund is worth a visit. From there I'd vote for the shortest way to Twyfelfontein – Opuwo – Epupa Falls (Kunene River). From there go east to the Caprivi Strip, to get to the Victoria Falls.
Now, most parts of this suggestion is on dirt roads, but they are easy to ride. Windhoek is not that important IMHO.
On the more easterly trip you might want to get to the most southern tip of Africa: Cape Agulhas. Some more highlights are the Swartberg Pass, leading to Prince Alfred, the road to Rhodes (Eastern Cape) and eastwards across the Naude's Nek Pass. At Rhodes have a (or stay overnight) at the Walkerbout Inn. Carry on to Underberg and conquer the Sani Pass (definitely not for road bikes), probably the most famous pass in S.A., have a at the Sani Top Pub. Travel north through Lesotho to Oxbow and maybe to Butha-Buthe, from where there are several exits back to SA. Now turn east, through Kwazulu-Natal to St Lucia Wetlands (World Heritage Site). Here I'd suggest to relax a few days, go and swim in the Indian Ocean at Cape Vidal, book a daytrip into the Hluhluwe-Imfolozi Game Reserve with local tour guides. Then either pass through Swaziland to Kaapsehoop, or along the Swazi border on the SA side via Barberton to Kaapsehoop. This road offers magnificent landscapes. Further north you can take the panorama route across different mountain passes (Long Tom Pass e.g.), Pilgrims Rest, Blyde River Canyon and on you go to Zimbabwe. There is a new Game and Nature Reserve in the south-eastern corner of ZIM which is still quite unknown to the general mass tourist scene.

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Only when we pause to wonder
do we go beyond the limits of our little lives.
(Rod McKuen)

I am also interested in a solo ride from Cape Town to Barcelona through the east coast in 2020. Just bought a KTM 790 Adventure, and I will thoroughly test it out in Australia before shipping it to South Africa. Just starting to do research on route, visa requirements, safety, etc.