In the past I've used an system dividing each province into estates held by the minor nobility of the realm. A province had one estate pr. level, giving a level 5 province five estates. These were run basically as a standard realm, but all income were divided by ten. This opened for relatively powerful vassals in cases where one noble had multiple estates in one or more provinces.

This was a fast and dirty patch to allow for a level of play my players wanted, but I've also seen other systems that used similar mechanics.