A DLC that adds more companion missions and "outings" would be just as good. Reason being all the companions share a faction from Rook's potential backstory, so there could be a greater emphasis on that interaction with Rook's backstory through dialogue and quests.
Although a prologue origin story like dlc sounds like a cleaner more likely approach to something like that, because a prequel can do more efficient correction and plot filling than add-on content later in the game.
More companion content would be great as well! Developing the companion relationships would also help peoplw connect to Rook in a way and have Rook feel like less of an outsider. I could definitely see a faction specific mission where Rook and that companion have to return and clean up some sort of follow up mess due to Rook's history and they can talk about what happened. I really liked the idea of a prologue to help develop more of what the game throws at us right at the beginning. Rook and Varric are close, okay how? Solas cleansed the lyrium idol, okay how? We're told a lot, and then it's never mentioned again!
