The only think I have to say about the Sussex dukedom is that the current holder has disgraced it so much that nobody else other than his descendants would want to use it.

Should it revert to the Crown, it will be regarded with the same disdain as the Windsor dukedom.

The monarch has the power to create new dukedoms. He or she isn't limited to those that have already been used by other royals. Something else could have been found for Louis when the time comes.