Many people love the movie, Big Hero 6 and are dying to see Big Hero 7! So many fans were upset how Tadashi unfortunately passed and that Callahan was a villain.
Theory 1) As I was watching the movie, I was so devastated that something so tragic happened in about the first 20 minutes of the movie. So, I thought there is no possible way Tadashi died. He might have gotten injured but in my opinion, I don't think he died. In the next movie at the beginning, I think Hiro might be doing something related with science (he seems to like science) or maybe with the microbots and Tadashi comes into the house and surprises Hiro.
Theory 2) I know, Callahan is a very bad man. But who says people can't change? Another way I think the next movie will begin is that Callahan comes back but he became a good guy after thinking all of it through. The other characters might not pay attention to him because they might think he is still bad even though he's not.
If you have any other ideas of how Big Hero 7 might start, give me some feedback! Thank you for reading!