As for the significance of the tree house itself, I think its story is that it is this huge monument that has been in Harambe for centuries and has been pretty much the home of the tribes that lived there. the tree itself could be called "Harambe" and it has served as a beacon for lost peoples. This would explain all the cave paintings and evolving displays within as tribe after tribe would come and camp there and add their own piece of history into the tree.
The design of the house on the tree itself could be an amalgamation of several african cultures. from the northern more moroccan features to east african, like Kenya, to the more tropical southern part like Gabon or South Africa. The tree house on top of the tree should be very colorful, reminiscent of say, the giant mask displays in AK lodge.