To be honest, I would need a secondary goal for overworld like courses have. It would allow creating a way to choose from different ways of how to complete game finally and it would allow secret world with easter eggs if finding a secret path, next regular world otherwise.
My another suggestion would be to add an object for moving to another course directly. This would allow, for example, to enter the secret world from several worlds and would allow to require you to find secret world from every regular world in order to beat secret world also.
A gate that disappears/opens once all regular courses(not castles) have been beaten would be nice too...