One thing I did to challenge myself was to only start wars via crusade. My first target was, of course, Estonia. It took a while to build the chapter house and crusade but when I did I took the province and rebel-farmed it for a while. The crusading option let’s you role-play it a lot and provides you with many religious fanatics to compensate for a lack of siege equipment. Plus, you really need to do all you can to expand your influence.