This is something which is mostly inspired by the fantasy world of Westeros but I can count a fair few different examples in history as well. That is the concept, which was frankly absent from CK2, of political manouvering during times of war. I'll here make two suggestions but if others have more, feel free to add.
Claim promise
This idea is that people can support your war or rebellion in exchange for a promise to a claim of theirs. That is that if a ruler goes to war he can vastly increase his chances of support in exchange for promesing to press the claims of his vassals, and after victory these claims are automatically pressed against the defeated parties. But claiments and rulers should be able to take initiative for this. It could also allow a character to capitalize on a ruler who has exiled lots of people by gathering these exiles to her and let them lend their strength to hers in exchange for returning home after a victory.
Example: The King of France wants war with England. The Duke of Anjou has a claim through his mother on the County of Kent while the Duke of Flanders has a half-brother in his court with a claim on the Duchy of Northumbria. So the King of France sends word to the Duke of Anjou that if the French king is victorious in the war, he'll reward the duke with Kent. The Duke's eagerness for the war skyrockets from being only mildly interested. Just after the war is declared the Duke of Flanders sends word. He says that if the King of France will add his claim on Northumbria to his war goals then the Duke of Flanders is all in. The French king agrees and suddenly two formerly disinterested dukes are very, very interested in seeing the war reach a victorious conclusion and so commits more fully.
Switching sides
Like it says it seems to me that nobles should have an oppertunity to actively switch sides during war as that would give more urgency and also allow for intrigue characters to have a serious angle to wage war from. Yes, a martial character is likely to win many victories but a intrigue focused character can wage war by turning the enemy's supporters against him and sow divison in their ranks.
An excellent suggestion of this is in my opinion the Byzantine handling of the Normans under Manuel I.
Claim promise
This idea is that people can support your war or rebellion in exchange for a promise to a claim of theirs. That is that if a ruler goes to war he can vastly increase his chances of support in exchange for promesing to press the claims of his vassals, and after victory these claims are automatically pressed against the defeated parties. But claiments and rulers should be able to take initiative for this. It could also allow a character to capitalize on a ruler who has exiled lots of people by gathering these exiles to her and let them lend their strength to hers in exchange for returning home after a victory.
Example: The King of France wants war with England. The Duke of Anjou has a claim through his mother on the County of Kent while the Duke of Flanders has a half-brother in his court with a claim on the Duchy of Northumbria. So the King of France sends word to the Duke of Anjou that if the French king is victorious in the war, he'll reward the duke with Kent. The Duke's eagerness for the war skyrockets from being only mildly interested. Just after the war is declared the Duke of Flanders sends word. He says that if the King of France will add his claim on Northumbria to his war goals then the Duke of Flanders is all in. The French king agrees and suddenly two formerly disinterested dukes are very, very interested in seeing the war reach a victorious conclusion and so commits more fully.
Switching sides
Like it says it seems to me that nobles should have an oppertunity to actively switch sides during war as that would give more urgency and also allow for intrigue characters to have a serious angle to wage war from. Yes, a martial character is likely to win many victories but a intrigue focused character can wage war by turning the enemy's supporters against him and sow divison in their ranks.
An excellent suggestion of this is in my opinion the Byzantine handling of the Normans under Manuel I.