The only normal route to drive right the way through would be to cross central Africa via Chad / Sudan. There are enterprising travellers crossing via CAR / Congo (both of them), even Angola but these woud all rate as far more serious. (could be a great adventure through if your up for the risk). 5 of the 8 major overland trucking companies sent trucks last year and they all went Chad / Sudan.
We travelled down in a Landrover and went Morrocco, Western Sahara, Mauritania, Mali, Cote D'Ivoire (not a good choice at present), Ghana, Burkina Fasu, Niger, Chad, Sudan, Ethiopia, Kenya, Tanzania, Malawi, Mozambique, SA. We went round lake chad from Niger to Chad - challenging on a bike - we only met one other bike going that way, lightly loaded KTM and even that went on the back of a 4x4 for a stretch. Other bikers we met all went via Nigeria / Cameroon to avoid the sand. Perfectly possible though - just harder work.
Lots of options - can go for the more risky routes in central africa, could ship from most of the coastal west African countries such as Ghana / Togo / Benin / Nigeria / Gabon - SA or Namibia and then head back north. Good luck.