A new character could be an alien or a being from another dimension who accidentally lands in their world. This character could have unique powers or technology that both confuses and helps Big Hero 6. They would have to learn to communicate and work with this strange new addition to their world while also dealing with any threats that come their way.
There could be a new tech - savvy kid who idolizes Hiro and wants to join the team. This character might bring in fresh ideas and a different perspective on using technology for good.
Maybe a future version of an existing character from their past, like Tadashi, brought back in some unexpected way, perhaps as an AI projection with all his knowledge and a new mission for the team. Another potential new character could be a rival scientist who initially opposes Big Hero 6 but later realizes they are on the same side when a greater threat emerges.