Looking for potential impact fantasy players in September roster expansions is a dicey proposition because of all the various factors involved.
1. Players have to be on the 40-man roster in order to be called up, and teams don't necessarily like to put players on the 40-man until they have to. Granted, some teams have some dead weight at the back end that they could jettison to create room. However, it means a lot of the bigger names might have to wait until next season.