Since having a flashback DLC would be irrelevant to the story of season 3, why not create a flashback DLC which introduces a new character?
Yeah, why not? I'd play it.
Like marks story from episode 2?
However they could do more interesting things with the dlc.
As some people wouldn't like playing as christa I would find it ludicrous to make a whole season based on her so maybe the dlc could be based on how she survived?
But that's just my hypothesis.