I agree, I'd like to see Ritchie especially. They could have done a whole flashback episode explaining why he left the group if they did it earlier. Since that would never happen I'd settle for a quick appearance in an episode. Doug's sister would be cool to see as well, even seeing Carrie's annoying sister would be kind of funny.
Anyways I think I've developed a pretty good picture of how the show will end so I hope they can do something (such as bringing back the characters above) to surprise me.