A Tron: Legacy sequel is already in development according to What'sPlaying [via Cinemablend] who have been "in contact" with heads at Disney. The site is reporting to have learned a few things about what we can expect from the possible next Tron installment. They claim that although Edward Kitsis and Adam Horowitz have not yet finished the script, they have a bag of ideas, five or six different storylines floating around. The site then adresses the question of why Yori/Laura [played by Cindy Morgan in the 1982 original] was completely left out in Tron: Legacy, as both Kevin Flynn [Jeff Bridges] and Tron/Alan Bradley [Bruce Boxleitner] were included.
What'sPlaying state that this was because the writers focused on what exactly happened to Flynn, so there was no point to have the Yori/Laura characters appear in meaningless cameos.
The site says that a few of the multiple story-lines will have heavier focus on both Alan and Laura. Furthermore, the cameo made by Cillian Murphy at the beginning of the film will likely expand to a much larger role, with both a real-life version of himself and a program version [with such a short cameo it would float that the actor is already committed this]. There's also talk that Kevin Flynn may not be a part of the next film, with Alan and Laura providing the call back to the original movie. This unsurprisingly is completely dependent to how Tron: Legacy performs financially and at the moment is mere speculation.