Campaign book and/or scenario pack would be the obvious choices. I think it will all depend on how well the game has done commercially. Clearly it's been a shot in the arm for Foundry's old Darkest Africa range, but I wonder if sales of the rulebook itself will support a lot of new supplements? I don't have a strong sense of how well the game has done in terms of profitability and sustained growth.
If you're looking for an interesting sidetrack, there was a very good article in wargames illustrated about playing Congo in South America either with Conquistadores vs Aztecs or Pulp style in the 1920s a la Lost City of Z. Highly recommended.
Ax