I'm thinking a victory might score you a "clue" which in turn may be cashed in to play a special scenario. Or perhaps a number of them. During these scenarios another player would be allowed to play the castles defences, consisting of animated statues gargoyles and the likes. As discussed before this will give the other player an opportunity to gun for the best models in the other players force. And for this instance I think that's all right. The purpose of this defence force is to repel any attempt to claim one of these plot items, and the defending player is wholly encouraged to build the force specifically to counter the attacking players group. This should be a challenge.
Optionally in a multilayer campaign if one player cashes in his clues to go for one of these missions other players who have enough clues are allowed to join in as well. "we heard rumours the blackwatch is going for the sword of Bendric, so we should move now!" Maybe even allow those forces to join in for a clue token less. That way you should get big epic scenes where everybody is trying their best to get that one item. Whereas during normal games one might retreat because the standard loot isn't worth the risk any more.
I playing with the idea that one of the warbands stats will be a something like path-finding or scouting. This is spend before a normal game ina blind bid. If somebody wins by a small margin its a territory raid, if its a large margin it would be a base camp raid and even or perhaps small difference would be a border skirmish. Left over recon points can then be spend on affecting the table layout. During a base camp raid you should be able to capture the above mentioned macguffins.
Some abilities of models will increase the scouting stat while they are present, naturally. And since this will be a game with many wizards this might also be a good area for divination spells to take effect. Maybe a divination spell allows you to name a number and your enemy has to say if he's going to bid above or bellow that number in recon points? I feel a campaign system like this is a great opportunity to have non combat magic shine more then it does in one of games.
Just Spitballing here, but there are gears like these forming in my head. I just want to get them out here because chances are a large number of them will get holes poked in them I didn't see before.