In the USA, if Disney were to add a new park, it makes most sense to add it at WDW where they have the land. There has long been talk about adding third park to
Disneyland Resort where they also have space for one (they have land nearby they own and can use).
IMHO the most well executed "second gate" Disney park is Tokyo Disney Sea. That would be awesome if that came to WDW or DLR. While I am not excited about a Marvel type of park, I think that is the direction Disney may be going.