I don't believe there is going to be a Big hero 6 Sequel. The theories of Tadashi returning as Sunfire are bogus! He died. They DID find his body.... That's why there was a FUNERAL. It was assumed Callaghan was burnt to crisp.
The movie doesn't need a sequel. Everything is now fine in the world of San Fransokyo. It would be silly to bring back Tadashi now, after all they'd done to avenge him. The plot escalated perfectly.
All Disney movies have a specific moral. Big Hero 6 itself was about coming to the understanding of grief. Knowing that they are always here.
This is also why Baymax sounded so human at the end... Reminded Hiro of Tadashi. I am also to believe that the spirit of Tadashi lives on in Baymax. Hiro taught Baymax many human characteristics such as 'Fist bump' and teenage slang like 'sick!'.
We can also understand that Baymax has a mind of his own (eg. Taking out his healthcare chip and fighting the evil chip to protect hiro and Callaghan's daughter).
I conclude my statement. Thanks for reading.