The basis for any Colonial / Darkest Africa wargame should, I think be based on the need of the major powers to exploit the natural resources of the relevant country, to seek greater influence in terms of geography and trade and to prevent other, rival, powers from doing just that.
One suggestion would be to use East Africa as a base for the scenarios. There is lots to experience there. So Kenya and Tanzania (as the are now) could prove to be ideal - plenty of Belgian, German and English colonials to use and the native tribes there would provide much scope for plot development.
Characters that may feature:
General Sir Norton Bottle-Blythe (Military Governor of Mabele Province)
The Hon. George Smythe-White (Landowner and part-time explorer)
Henry Cabot Adams (American importer / exporter)
The Hon. John Hector Neville Winstanley (Noted explorer and adventurer)
Jean-Luc Flambard (Belgian mining engineer)
Prof. Louis Marcaut (French archaeologist)
Angus Campbell-Bannerman (Scottish mining engineer)
The Rev. Peter Cecil (Missionary)
Sir Andrew Parker-Leake (Colonial administrator)
Gert von Stauffenhalle (German diplomat and spy)
Needless to say, the above could all have competing objectives. Certainly Gert von Stauffenhalle is inciting trouble amongst the Umabele tribe, arming them with rifles and bombs. What are the Belgians up to and why do they need over 100 Askari to guard their mining operation? What about Angus Cambell-Bannerman's own mining exploration? He was up until six months ago a Captain in the Kings Own Scottish Borderers, so why come to Africa in such a hurry? How about Henry Adams - why is he working so closely with the French archaeologist Professor Marcaut?
Also, rumours abound about the Ngorongoro tribe inland, they are becoming restive since the English party of explorers have discovered a previously uncharted plateau in the jungle. Perhaps a detachment of the 17th Norfolks, under Captain Roderick Courtney, should be sent up river to Kingora in order to investigate...