I had an idea for how to represent the nobility ranks below Baron. There would be 3 new building trees, Manor(Castle), Manor(City), and Castle(Small). You could build these and grant them to unlanded nobles as Honourary Titles. They would work like Baronies except they are buildings (i.e a lord of a manor could marry, deal with sucession, and everything else a landed feudal noble could do). The only limit to the number of these "mini-holdings" would be the province "size" or max number of mini-holdings.