Certain things have to happen before Disney even considers a new park:
-More hotels (shocked at how they haven't added more, and they should really add a large/value moderate hotel)
-Water Park (not really needed, but surprised Disney hasn't attempted it considering how hot it can get in SoCal)
-Parking situation (even with the new garage they still need more parking closer to the resort, something has to be done BEFORE they even work on a new park)
-Disneyland East (Tomorrowland & Fantasyland need updates.)
-DCA still needs to fix that mess called Hollywoodland
However, I don't think Disney will build a new park in the stadium location. The Toy Story parking lot is a lot closer and easier to transport guests between, not too mention that they can connect it to (hopefully) the Eastern Gateway project if it ever becomes reality.
Sure they might have some old plans (WestCot, DisneySea, etc.) already, but I'm sure they will create something new just to fit whatever IPs they want.