This mod adds a form of region trading to the campaign!
When you assign a governor to a province, you will now have a choice whether to assign the character as governor (as usual) or to create a new vassal in control of that province (this kills your governor - as if he were to become the head of the new vassal state).
You will only be able to create a vassal state if the faction that would be liberated from the province is either dead or already a vassal of yours. Unfortunately, at the moment, there is no way to tell in advance if this is the case. If you begin as West Seaxe, assigning a new governor to Dorseate should allow you to create a vassal, however, your other starting provinces are all true West Seaxe provinces and so there is no option for you to create a vassal for them.
You can play with one of two versions of the mod:
The first, the "Vassalise Province" mod, simply adds the aforementioned mechanic to the campaign.
The second, the "Vassal Kingdoms" mod, adds penalties to factions which own too many regions; it increases corruption and owning above 20 regions reduces the loyalty of your other characters. It is designed to incentivise the player to make use of their ability to create vassals and (hopefully) prevent them snowballing through the late-game.
Potential changes:
I will update the flavour text for the vassalise option once I am feeling a little more creative!
I am open to suggestions for penalties related to region holding - it is possible to create large diplomatic negatives instead of reducing character loyalty for example.