Can we please move on from the discussion wether we should have a passage between Kongo and the great lakes? Ever since it was suggested it is being held as a scapegoat why east africa shouldn't be expanded.
Frankly, looking at the geography of contemporary Tanzania, Kenya, South-Sudan, Uganda, Rwanda and Burundi, I cannot see a reasoning for it not to be included, accessible from the Swahili coast. It actually seems very far more passable than other places already on the map, like the Rockys, Persia, the Alps or even contemporary Sudan and Ethiopia. I have only read once why these areas would be implausible to march troops through other than: it just didn't happen.
Could I also suggest Malawi for consideration (as that would allow the Maravi Kingdom to be included)?