As a wise man once said, "You don't make something as long as Homestuck by thinking about what you're going to do when you finish Homestuck."
Assuming, Lord willing, that I'm able to complete Runewriters, I will be 40 years old at the youngest. I invented the comic in college and I started creating it when I was about 23; I'm not the same person I was when I started, and I won't be the same person when I finish it. I can't tell you what I'll want to do then. I have other ideas, certainly, but the stories I create now may or may not resonate with me then. Maybe I'll be satisfied with a completed comic and want to pursue some other dream; maybe I'll have a new story I can't possibly predict from where I'm sitting now! I don't really think about it too hard; it's too far in the future to say.