It can be recalled that although Hiro is an inventor, Baymax was not his invention but that of his brother Tadashi Hamada, and was primarily meant to be a care provider.
Unfortunately, Tadashi died in “Big Hero 6” after an explosion during the science expo. But accordingly, he may have survived it as well just like Professor Callaghan, who turned out to be Yonkai, the main villain in the first film.
Based on speculations, just like Professor Callaghan in the first movie, Tadashi will still be alive and shall return as the villain Sunfire. In the comic books, Sunfire is an original member of the Big Hero 6 and his superpower is his capacity to withstand excessive heat.
Apparently, one of the many inventions in the venue shall be able to save Tadashi and he will return as Sunfire. He also has the capability to shoot fire.
When he comes back, he will be battling it out with the Big Hero 6 led by Hiro and thus the brother versus brother conflict.
Since Tadashi was the one who invented Baymax, speculations are also rife that the robot might be initially confused on who between Hiro and Tadashi he will side with. However, when the robot sees the ill-intention of Tadashi as Sunfire, he will come to his senses and stick with his five other colleagues in Big Hero 6.