Similar to how lords currently execute rebel lords, they should also start executing lords belonging to a faction that has no fiefs after 21 days the loss of a faction's last fief. I think that this would be an easy and practical solution for TaleWorlds to add in a short amount of time.
As an addition, similar to how you receive mercenary and vassalage offers by "mail" (on the sidebar), lords without fiefs and lords that have fiefs on the border with your faction should also send an offer to join your kingdom when the 2 factions are at war. The "mail" should say that the lord will be waiting in x castle/town for 7 days to swear fealty if you click 'accept' on the mail notification. This can even be added as a quest to the journal.
As an addition, similar to how you receive mercenary and vassalage offers by "mail" (on the sidebar), lords without fiefs and lords that have fiefs on the border with your faction should also send an offer to join your kingdom when the 2 factions are at war. The "mail" should say that the lord will be waiting in x castle/town for 7 days to swear fealty if you click 'accept' on the mail notification. This can even be added as a quest to the journal.
