One route addition which is absolutely outstanding is West (Central) route. Head through Nigeria, Cameroon, Gabon, Congo, DR and Angola.
The West route is also the cheapest and least populated so you end up bush camping 90% of the time. The road conditions vary from desert, bush, oil plantations, and jungles! Plenty to keep the adventure alive...
This route to me, is the best - except for perhaps crossing Central Africa (Cameroon, CAR & DR)...