retelling of Heart of Darkness
> there's a lot of potential in the basic idea of a retelling of Heart of Darkness in a post-apocalyptic
> future SF setting. It could be done seriously well if someone wanted to try.
I can't think of an example off the top of my head, but most likely it has been (tried, not necessarily done well). _Apocalypse Now_ is on the edge of "post-apocalypic future SF"; there's little thematic difference between the contemporary-guerrilla-war setting of AN and many visions of the post-apocalyptic. _Escape from New York_ has elements of the central HoD trope, though it's essentially a different story (closer to Kipling's _Kim_, for example). And so on.
HoD and its analogues reiterate a standard dichotomy of European imperialism from the 19th and early 20th centuries: the dominant middle class, fearing that its powers are waning, wants to consume (destroy / enslave / learn from / screw) "native" peoples, whom it fetishizes as energetic / spiritual / etc; but it's also worried that it will find them too attractive and "go native". It shows up in any number of novels of the period. In James' _The Ambassadors_, you have the HoD plot except the tempting-woman figure is French (but spoiled by having been married to some Eastern European aristocrat) rather than Congolese. In Trollope's _Barchester Towers_, she's English (but spoiled by having been married to some Italian aristocrat). In Wharton's _The Age of Innocence_, she's American (but spoiled by ... you get the idea).
A great many of our SF tales are just imperialist nostalgia.