This is hard, Anon. I mean, I think Kacchan growing up, he wouldn’t change so much physically. Maybe grow taller I guess. He already has a great body lol and his hairstyle is really hopeless… plus his costume also has a great design. It’s quite hard to top that haha
However, I think most of his growth would be for his personality. Unlike some of us fans think, it never did seem to me that Kacchan would even consider becoming a villain. So, growing up, he’ll mature and learn how to become an actual Hero with aligned morals and vastly improved cooperation skills.
By the way I got some fun fact for manga readers at the end of the tags. (might be considered spoilers for anime watchers only)