Since the three initial campaigns for the three starter factions are in the rulebook (and it's been suggested that they're faction specific) then the assumption is that new warbands will come with their own campaigns. However I'm not sure how a campaign would fit into the card decks for the non chaos factions as they won't have miniatures in the box with them to make room for campaign rules booklets. Perhaps they'll put the campaign on a group of cards (similar to a lot of the old Rackham scenarios for Confrontation and Hell Dorado).
It might be assumed that non chaos factions simply pick a campaign from within the rulebook and, until we get our hands on the cards, we simply won't know.
This post is mainly a placeholder for later discussion but also to stir up interest in the subject now. Whatever the situation is I'm sure we'll make it work somehow.
A further thought. It's not much of one but I've been mulling over it for days now.
The core rulebook contains 2 campaigns for each of the six core factions and one more for each of the non core factions (including the other two chaos factions).
If they are going to introduce new factions then it will probably be in the form of an expansion with the campaigns contained in the rulebook again (kind of like a "season 2", if you will). This means we probably won't be seeing any new factions any time soon but it also means we get plenty of time to enjoy the existing ones without havign to worry about any new interlopers coming and stepping on their toes.
Can only guess ofc , but at a guess they may just say "X mortal gang uses X campaign tree( and just modify slighlty the narrative in your heasd)" or there will be KT type expansions, " elites" or " Gang War book I " etc