Big Hero 6 was an origin story. Fred even said as much in one scene. Whether or not we want or need one, I'd imagine we're going to be getting one. And that's just fine with me. I loved the movie, and there's still a ton that they can work with. They have a solid foundation with some interesting characters, most of whom were pushed aside in the first film, and with an interesting setting.
That said, part of what made the first movie great was that it took a rather standard (and tired) story (superhero origin), knew that it was working with a story many were tiring of, and used that self-awareness to have fun with the idea. It also helps that it tried to be more than just about the hero himself but actually tried to relate to those in a similar situation (i.e. those who have lost a loved one recently). And all the clear attempts at getting the geeks in the audience to self-insert themselves was a nice touch.
So, in the end, if they can expand on the foundation of the first film, both narratively and thematically, I'll definitely be happy with any sequel they come up with.