What about a Kongo Empire turning in Brazil-equivalent, being independent at the beggining of XIX century?
That's dumb, Brazil's strength is not related at all to the barely populated expanse of the Amazon, an expanse of jungle will not help a grand power grow.
Despite the usual AH idiocy of looking at a space filling area on the map and conflating it with potential power, the vast majority of the DRC's population live in either the downslopes of the Angolan Plateau in the south west (this is where the historical Kongo kingdoms were located) or along the mountain borders of the great rift valley.
A Kongo Kingdom/Super Angola that did own the Congo basin would be a powerful country, but it wouldn't be the DRC, and would hardly be stronger than Nigeria or South Africa in Africa alone, much less a great power. It would also have enormous problems assimilating the eastern ranges that are actually populated.
Secondly everywhere in africa has only seen very recent large population growth, looking at the big numbers now has little baring on strength back then even in the immediate post-colonial era (indeed population growth faster than economic growth beggers the country).