What if Monkey D. Dragon is just like garp and luffy, outside the seriousness? I think it wouldn't be a big surprise. I think he's just more in control with his serious side, unlike the other two.
I don't care how his personality is, we all know he will be epic But so far, he seems like the more serious kind of person. I think he will be like Garp or something. In other words, he's serious, but he can still have his moments