to get the boring out of the way: Problem Sleuth. a lot of Homestuck readers straight up don't get the key bits of what Homestuck is about because they don't get that Homestuck really is just a reboot of the Problem Sleuth concept.

but if we're talking about hard sells... i think the ways Homestuck and Neon Genesis Evangelion end up hitting on many of the same themes and motifs re: adulthood and maturity versus childhood and stagnation are really fascinating. people naturally compare The Homestuck Epilogues and Deltarune a lot these days but I think Evangelion and it's relationship to the Rebuild of Evangelion series is a super essential look into one of the early forerunners of the "epiquel" concept, even if the Epilogues did manage to beat the release of the final Rebuild film by a couple years. Anno pulling the rug on everyone at the last minute to reveal that one of the most enduring gay love interests of the last 30 years is actually the protagonist's dad so he could hook the protagonist up with a random woman he's barely talked to before instead really explains every question you could ever have about the Homestuck sequels, if you kind of squint your eyes and tilt your head and make sure you're just a little fucked in the brain too