Planning a route to meet this goal could be challenging! If your goal is primarily a circumnavigation, going east/west in the northern hemisphere is pretty sensible. The challenge comes in including Africa, South America and Australia in a landmass-contiguous route. One option would be to follow motorcycle adventure-tourer ted simon's route (detailed at jupitalia.com). Basically he travels from England to Africa then from Johannesburg to Brazil. Loops South America the long way (i.e. southbound on east coast, then north on west coast to the US). Then a tour of Oz, eventually returning to England overland, starting in India.

There are certainly tons of options, depending on what countries are on your 'must see' list.