When you have that kind of buy-in for a sequel and the story left the ending wide open, you almost have the duty to bring the fans together at least one more time. After the movie won an Oscar for Best Animated Feature, there was plenty of talk behind the stage about getting together for the second part of this story.
Big Hero 7 might have been born at that time, but there was no definitive talk at that time. Stan Lee was interviewed about the future movies that will be worked on and besides the Black Panther, Doctor Strange and Guardians of the Galaxy, Big Hero 6 was also mentioned.
That was confirmation enough for fans from all over the world as they were excited to possibily see Baymax and the rest of the crew come back for Big Hero 7 at some point in the future.