Representative Referendums are already there for Lords and Dukes, but it disallows the rest of Nobility to vote directly. Still, that can be used more. Or perhaps we need some system where all of Nobility gets to vote, but Lord and Dukes get more votes dependent on the amount of Knights in their region.
I do believe perhaps open estates or wild lands should give no tax income (Or go to the Ruler with a hefty penalty, though that might make Rulers very very rich), so a Knight actually benefits the Lord. Though, that may introduce balance issues, I don't know.