Here some ideas for a Hiro stories and you can do a Tadashi idea with them if you want

What if Tadashi survived and was just in a temporarily coma ? when he woke up Hiro told him everything about being a super hero and Callaghan revenge and how Baymax sacrifice himself to save him and Abigail. Tadashi was in the Big Hero 6: Series, this idea is a AU one.

Another idea

What if Obake made a new San Fransokyo somewhere else ? He just use Hiro's amplifier as a distraction to have time to take Hiro with him and make him his Student, when they got there Hiro try to escape but was restrain by Momakase and was taken to a room to rest in, Obake who real name is Bob Aken have made the decision to have Hiro brainwash or put fake memories in him. (It can be that Obake makes Hiro think that he is his son if you want) Aunt Cass found out who Big Hero 6 is and she demanded them to tell her everything and Karmi somehow found out Hiro is captain Cutie th went to his friends as well. After that they begin searching for Hiro and Obake with the help of Alistair Krei and Professor Granville. Will they find him and bring his memories back ?

Another Hiro idea

Episode City of Monsters

What if Di, Liv Amara clone knocked out Hiro after he got through to Karmi, she and Chris her assistant put the biotech on Hiro neck. Unknown to Di and Chris the real Liv Amara woke up and got the remote from her clone who was happy that she alive, but was shock that she was disappointed of what she have been doing she press the remote and return Karmi and the others to normal. Di was hurt and angry she and Chris made their escape with the unconscious Hiro later the rest of Big Hero 6 come but was too late, Karmi later find out that Hiro is captain Cutie and the rest of hero 6 identity. Will they find Hiro and turn him back to normal with the help of the real Liv Amara and Alistair Krei ? Also Aunt Cass somehow found out about everything and want to help to find her nephew.

Another Hiro idea

What if Tadashi never went into the fire ? He and Hiro had a fight, Hiro ran away and was captured by Obake who was watching him for a while, he wanted Hiro to be his Student. What if Tadashi took Hiro place as big hero 6, when they find Hiro he didn't remember any of them, (it can be that Obake turn him into a young Child or he brainwash or hypnotized him to forget everything) will they save him and bring back Hiro memories ?