In addition to those stories linked to above, more on Maggin's take of Superman's future and ultimate destiny can be found in
Superman #400 (online at this site, just click on the bottom-most left-most thumb on every page) - this issue also shows Steranko's prediction on the fate of the Universe, while
Superman Annual #10 (also online at this site, in the 1980s part 3 section) shows Maggin's prediction of the same.
Then there are other takes: Like the Superman's costume story in
Kamandi;
Action #215 where Superman travels to the future and meets his descendent Craig King (checkout the cover at
www.comics.org); other descendents like Klar Ken T 5477 and Laurel Kent (The first appearance of each is also on this site. I've often wondered what their relation is to each other - cousins perhaps?)
Superwoman is another future character whose existence is a result of Superman. Not a descendent of Clark, but certainly descended from the Superman Family.
I've also read various stories from the 1960s where Superman ages into a doddering useless figure and ends up in an old folks home; and there were the back-up features from the 1970s or 1980s that showed Superman's son and grandson taking up his mantle. When taken in combination with the various imaginary stories and Earth-2 stories where Superman and Lois marry - and some where they have super-powered children - it's obvious that Lois was the matriarch of all these super-descendents.
Jerry Siegel summed it up pretty well back in 1938: "A physical marvel, a mental wonder, SUPERMAN is destined to reshape the destiny of a world! It is not too far fetched to predict that some day our very own planet may be peopled entirely by SUPERMEN!" - which brings it back full circle to Steranko's story.