In this day and age, if a movie does well enough, it's common for the studio that made it to announce a sequel right away. Animation studios are no different; DreamWorks Animation is the best example of this practice, as they tend to announce a sequel (or sequels) as soon as a film does good business. Most recently, Warner Bros. Animation announced a sequel to The Lego Movie as soon as the film started earning big bucks at the box office. Other studios, like Pixar, take their time with announcing sequels to their films (just recently we learned of sequels to The Incredibles and Cars). Surprisingly though, the studio that hasn't released many sequels in its long history is the oldest of them all, Walt Disney Animation Studios.
In the more than 90 years that it's been in business, WDAS has only produced two sequels, The Rescuers Down Under (1990) and Fantasia 2000 (2000), and both of them are rather underrated and not very well known among the films in the canon. (Note, we're not counting the endless amount of 'cheapquels' - e.g. the various Aladdin, Cinderella and Lion King sequels - because they were made by DisneyToon Studios and are not official.) So, it just seems a little bit odd that in an era where sequels proliferate mainstream film, a studio as big as Disney Animation hasn't taken that route yet (for the record, I'm not complaining, I'm just stating it's rare, since most mainstream animation studios are doing it). However, that could hopefully change, since I think Disney has two films that have the potential to start great franchises: Wreck-It Ralph and the upcoming Big Hero 6.
Now, many people hear the world "sequel" and start complaining about the lack of originality in the industry. And while I agree that we need original projects, revisiting old ones can be as satisfying as having a good original property, if the sequels can tell a good story. So why Wreck-It Ralph and Big Hero 6 instead of Tangled and Frozen? Well, first of all, a sequel for Ralph is reportedly in the works already, so it may (hopefully) be announced soon. But, even discounting that, I think both films have worlds and characters that can be expanded in many different and interesting ways. In the case of Ralph, the video game world is a gold mine of stories where many others can be told from many different angles while adding fun new characters. In the case of Big Hero 6, I think it has even more potential since it's a Marvel-based superhero film, and the possibilities for new adventures are endless. In the case of Tangled and Frozen, because they are fairy tales, I think their "happily ever after" finales pretty much end the story; there's no need to see where the characters are going next. They have pretty finite endings so their sequel potential is more limited (not that it's out of the question - especially for Frozen - but I think sequels to these films would feel forced).
But going back to Ralph and Big Hero 6: both films have great worlds, great premises, great characters that can be expanded into future instalments. Of course, this should be done only if Disney has great stories to tell and not just for the sake of money. If done right, Disney can have their own Toy Story or How to Train Your Dragon franchises and the results could be truly spectacular. We'll just have to wait and see - but, if done right, seeing many adventures for Ralph and Baymax would be a very good thing.