Though these probably falls into the realm of overcomplicating things, they also add both motivation for someone of upper station to duel someone of lower and create the notion of "champion":
a)Fight not only for occupying the title, but also for appointing the title holder. That way Duke Kepler duels Duel Weakling for his loyal Vassal Puppet to inherit the Weaklingstan Duchy, but risking losing Keplerstan Duchy to Duke Weakling.
b) Want it moar complicated instead? One noble writes a "contract" which other one or three noble(s) will have to accept for the duel to happen. Two nobles duel and the title goes to the winner or the winner's sponsor.
P.S.: Tom, please don't hate us for dreaming.
These are all fine roleplaying ideas. They can be enforced in game. I don't think new mechanic is needed.
