I think Jack was the life & heart of Borderlands 2. People tout Borderlands 1 for its impeccable, unprecedented atmosphere & full commitment to its themes, however, more I think have fell in love with Borderlands 2 not even utterly for its story, but for The unique experience that Jack creates for the world.
Grungy, dark humor, serious post-apocalyptic games & media feel a dime a dozen. But Jack's presence cuts through the very intense dark realities of Borderlands 2 & keeps you curious about his next steps.
A very good yet true-to-form expansion of Jack's character could enrich the world & add depth while preserving the main source of levity available to the series.
It must be done superbly & with the same comedic strength as Borderlands 2, however, simply another dark or apocalyptic space game could easily lose people's interest for many reasons & will have a hard time generating a persistent line of connection to the hearts of its players.