Title: New button for single-duchy dukes: "Seize the Throne"
Summary: Basically, dukes of the last duchy in the realm are actually less directly powerful than if there were another duke: they can be banished, bullied or whatever and can't have any recourse. While it is understandable that seceding the duchy is destroying the realm uselessly, there is a middle ground: just let the duke take the throne, instantly getting rid of any banishments, making him the king of the realm and letting him keep his duchy. This incentivizes rulers to either distribute power among multiple dukes or be forced to trust their single duke implicitly. The old king would still be a royal and wouldn't be banishable, giving him an opportunity to rebel and take both the throne and instantly banish the new king should he win. Dukes should be attempting to expand their domain, not shrink it so that they can still hold some form of leverage. The current system makes no sense.
Details: When dukes are otherwise prevented from forming a new realm and are the only duke left in the realm, they may seize the throne, bloodlessly becoming the new ruler.
Benefits: Balances the game a bit more. It doesn't really make much sense for the final duke in the realm to be unable to cripple the realm to the extent that a less directly-powerful duke would be able to. These kinds of people shouldn't be able to be bullied and making your realm single-duchy shouldn't be an insurance against treason.
Possible Exploits: It might be that some dukes would use this as an alternative to a rebellion, but the fact that the ruler wouldn't be instantly banished is a downside that might prevent that.
