My reading of the ending of Homestuck always hinged on the idea that it was a happy ending because they were able to escape being in a story. They spent basically heir entire childhoods in situations forcing them to have arcs and grapple with narrative consequences and they could finally be free from that. With that in mind, I honestly think the best way to continue the story is through the lens of a depressing fanfiction forcing the characters back into the plot that they escaped.