Its is chronologically more accurate to depict Tyrannosaurs in F-14s than it is to show a T-rex fighting a stegosaurus.
That's probably the most famous follow-up, but there have definitely been a lot more fancomics about grown-up Calvin, although I can't remember others by name. In my experience, most Calvin and Hobbes fans really dislike them for some reason.
The issue with "grown up Calvin" is just that: Calvin is grown up and that isn't Calvin.
There are so many things that pretty much foreclude a "fan continuation". The largest is that usually the fart-huffer trying to grave rob C&H can't help but having their smug subversion of the thing they are graffitiing and that removes all charm and whimsy.
They also never grapple appropriately with the nature of Hobbes where he doesn't seem to be more than stuffed toy but then things happen in the comic that should be impossible if he was truly inanimate.
The one's that don't completely shit all over the premise by having Calvin "grow out" of his imaginary friend and still have Calvin and Hobbes interacting while he's older ruin the core premise by ever interaction injecting a bitter reminder of what growing up took from us. The magic of reading Calvin and Hobbes is adults reexperiencing that (and for kids, relating to it) so an adult calvin completely ruins that timeless appeal.
The only possible way you could even hope to provide comics that aren't just crude imitation that continuations of the original run, would be to have post-retirement Calvin having idle-old-man adventures with Hobbes.
Calvin doesn't work when he has serious responsibilities he can't just duck out of, and retirement home Widower Calvin would fit that bill nicely.
The fact that they like the tranny fancomic to the exclusion of all the others is pretty telling.
they are not fans they trannies.
But seriously, break this down.
See what I mention above where Calvin only works when he has no responsibilities. he is self-centered, too dumb to know how dumb he is, no forethought. This describes the average troon.
A sociopathic narcissistic AGP Troon expecting the world to pay for his worthless tranny existence the the closest thing to a real understanding of the character.