I would probably be very excited to see Luffy change, in whatever direction =) I like this kind of dynamics. But that said, I think Oda won't do that, cause there is a particular direction he wants his story to follow. While it becomes more and more oriented towards taking on realistic social issues, I also I think Oda wants his manga to remain positive, to give people uplifting feelings of hope. That is why I don't expect him to change Luffy much. Also, literally millions of people will be upset if Luffy changes too much too fast, I think that is a huge factor too for Oda.