will = be going to make a good teacher = 良い先生になる in the future = 将来に
「彼は、良い先生になるでしょう。」 He will make a good teacher. He won't make a good teacher. Will he make a good teacher? Yes, he will. No, he won't. What will he make in the future?
He is going to make a good teacher. He isn't going to make a good teacher. Is he going to make a good teacher? Yes, he is. No, he isn't. What is he going to make in the future?
