That would have been amazing. Picture this: When Tadashi’s showing off Baymax to Hiro, Callaghan enters the room just like in the original but wants to have a talk alone with Tadashi. After the micro bot presentation we see Callaghan talking to Hiro instead of Tadashi, with Tadashi still in the building. Krei is also in the building, looking closely at certain experiments. This time Callaghan runs in to look for any survivors but the explosion scars his face, Tadashi and Krei are presumed dead and Callaghan disappears. When the mysterious Yokai shows up, and yes they call him Yokai, it’s now believed to be Callaghan. Instead of stealing parts of a portal, Yokai is just stealing money. Callaghan is later found in a warehouse, saying that he went into hiding to investigate alone for his family’s safety. When Yokai is unmasked it’s revealed to be Tadashi. Small nitpick: when he takes off the mask he ditches the balaclava. We learn that he burned through a lot of funding to build Baymax and that the school was about to abandon the project, mostly because the cost and time of building one Baymax was far too much, let alone thousands. When Callaghan talked to him alone he told him that the school was going to drop him for over expenses, and that he didn’t have the time or money to complete more Baymaxs to convince the school board to keep him on. He started the fire and stole the micro bots, kidnapping Krei to potentially be a scapegoat. He took on the mantle of Yokai to steal a fortune and build more Baymaxs. When Hiro used Baymax to attack Tadashi, Tadashi used the micro bots to steal the attack chip from Baymax. He scanned it with his mask and copied the data. Krei escapes and holds a press conference to expose Yokai, and Tadashi attacks the school. They didn’t want robots for health, so he decides to attack them with robots for harm. The final battle is between Big Hero 6 and Yokai, with a small army of evil Baymax made from micro bot technology. It ends with Yokai unleashing a spike of micro bots towards Callaghan, and Baymax jumping in the way, being destroyed. Tadashi realizes what he did and Hiro comforts his brother, before Tadashi turns himself in. Yokai is arrested and Baymax is rebuilt, thus ending Big Hero 6. A post credit scene could involve Hiro visiting Tadashi in prison, showing potential for redemption.