I thought "Tron Legacy" did a pretty good job of passing the torch actually, were there flaws in the story? Yes. That's part of the problem of what people have been talking about. These types of movies generally don't work out, because there is so much going on in them that a script can become sloppy.
As for cast members, I really like Ackroyd's suggestion a few months ago about Alssya Milano and Eliza Dusku. I think there was a picture of Eliza in a GB uniform with a proton pack on twitter or something. There were rumors of Seth Rogen playing a grown up Oscar late last year, early this year that were never confirmed for denied. Steve Carrell's name has been mentioned a few times (which I love). Here are my suggestions:
Steve Carrell as the new Egon character
Eliza Dushku as the female character/potential love interest
Michael Cera as the new Peter character
Mos as the new Winston character
Maybe you toss in a couple of new secondary characters that are friends of these guys.