Don't expect much from DLC. Matt and Trey are huge gamers (which is why they wanted to create a proper SP game), and know what gamers on the whole like/dislike (as evident in this game's mocking all the game cliches out there). In the very same Q&A session where they said their original script for the game was too long and needed to be cut down significantly, when asked if that meant DLC, one of them (I want to say Matt) said "F*** that!" (referring to cutting content in order to sell it as DLC later on) and then they went on to say the cut content would most likely end up in the show in some form, rather than DLC.
If anything comes from DLC, it'd likely be costume/weapons packs, and that'd be it. Before Ubisoft aquired the game, preorder bonuses were announced which were ultimately changed, and have little to no appearance in the game as it is now (completing my second playthru last night, the only thing I've seen was the Mysterion outfit in Kenny's closet and Professor Chaos' board of trying to figure out who Mysterion is (although the latter wouldn't have been a part of the costume pack Xbox users were to get)).
Also, remember that unlike most games out there, Matt and Trey play a HUGE role in this game (playing the same roles they do in the show), which is a large factor for why it took so long for the game to come out. As such, story DLC would take a minimum of one year (if we're lucky), and would be subject to all the delays the main game was, for the exact same reasoning (Matt and Trey's control over everything, their only working on the game as a side project between seasons of the show, etc.).