Don't ask what the classes are, because I have no idea for most of them. I guess Knight Helmet is a Page, Slipper is either one of the stealing classes or a prince(because heart-breaker lol), Chainsaw is a Bard, Treasure Chest is a Knight, and Journal is a Seer?? I was going by what was in the comic(which tends to be very vague), so bear with me here.
Here are some explanations for some of them:
-I figured it would make sense for Caramel Cube and Globe to be the obligatory aspects for the session. I can imagine Globe getting really shaken up by the responsibilities of being a Time Player. Caramel Cube being a Space Player mostly came down to the quest being a team effort, and she is the more well adjusted of the 4 PPT2 leaders, so it only makes sense. I imagine these factors combined with the Knight being TREASURE CHEST of all people would cause trouble.
-One would say Chainsaw fits rage more, but while he isn't the most approachable, I wouldn't say he is really defined by anger in the same way Plasma Ball is. Roles are said to be something that players have to work with, so having rage powers on top of also happening to have anger issues and shit to deal with would be Plasma Ball's challenge.
-Assuming Breath is themed around changes, Chainsaw would fit the bill. The formation of Team ] saw many things change for the worst, after all. I can see Chainsaw trying to munchkin his way through the quests and inadvertently break the game, rather than using his role for something more beneficial.
-Mind seems to be about choices of others. Bottle is a mediator, and her story in the merge involves struggling to change the minds of other people and its consequences.
-....Okay, Slipper's was really just a pun. To be fair, some of the roles in canon are puns, especially the ones of the heart players(role playing and breaking the hearts of others).
-Blood is implied to be themed around connections(seeing as it was pretty much tailor made for Karkat), and Treasure Chest's connection to his team is a big part of his story. That and I really really wanted to give him Karkat's role. If you know, you know. (also TC's paladin cape looks like the knight cape and it would be funny if the knight wasn't actually KNIGHT Helmet)
-...I think Journal as a Seer of Doom speaks for itself....
-Some aspects seem to also manifest in the figurative version of their aspect. Light is consistently associated with knowledge, so it only made sense to give it to Post-It Note, who has plenty. He isn't a particularly fortunate person, but the journey for these players aren't that easy anyway soooo uhh it cancels out.
-As for the rest... I don't really have much of a reason for them. For the cast, I just chose the characters who I think would work in a session the most, with half being Team H/? members and the other half being from Team 47.
Might consider the possibility of custom classpects.... I mean... Some of the canon aspects seem to be very much made for certain characters.... I really like the idea of SBURB being customized for each session, so there would be some different mechanics here and there. That said, I am more of an alchemy type of guy.
I don't really have much of this AU thought out. There are a lot of factors you have to consider when making an AU like this, and I am someone who is really into the gamey aspect of the comic. I imagine the parameters for these 12 would be much different, at the very least.