It's not a pretty picture!
#NABF18, "Holidays of Future Passed." First, because it was an entire episode set in the future (as opposed to the future being part of a framing device or a Halloween special). And second, because it had a tie-in with an earlier episode set in the future.
In 2005, episode #GABF12 "Future-Drama" featured Professor Frink foretelling Bart's future as a high school senior. Bart was dating a girl named Jenda (voiced by Amy Poehler). He proposes to her after the high school prom, but she turns him down, fearing that he will not be able to provide for them.
Flash forward to last week, when we learn that Bart and Jenda had two sons. And they must have married, because they have already divorced. And, as Bart learns when his sons arrive to spend Christmas with him, she has remarried, as well.
Clearly, Bart rarely sees his sons. He is surprised when they show up for Christmas, and takes them straight over to his parents' house to dump them. Where Bart's relationship with Homer was typically hostile, his relationship with his sons is simply missing. They seem like mildly aggravated strangers. (Who can blame them?)
Bart has literally never left his familiar school room. The school was converted to apartments, and he lives there still. Good old Room 203 hasn't even been made over - it still contains the desks, pencil sharpener, and other classroom accouterments.
In this, Bart shares his fate with Skinner, who has stuck around as the landlord. A somewhat disturbing idea, to be sure. Skinner has always been portrayed as the classic social failure, the man who still lives with his mother, and the ultimate mama's boy. (I mean, except about how he's actually a war vet named Armin Tamzarian. Ignore that.)
Surely Bart deserves a better fate than Skinner? Unfortunately it seems that underachievement really isn't as useful a life skill as you might think. If Bart was a rabble-rouser and a scamp as a boy, he seems to have lost that spark in his long slow slide towards adulthood, depression, and apathy.
Homer was an unmotivated slug, too, until Marge got pregnant. Bart's pending birth is what propelled Homer to hire on at the nuclear plant. Bart obviously faced the same situation, but instead of knuckling under and getting a job, he apparently just gave up on life entirely. Strange to think that, compared to his son, Homer is quite a success.