Nothing's going to happen beyond the new land or mini-land at Disneyland until the resort addresses its major transportation issue. Disney faces a huge barrier to additional expansion at the resort - the lack of space to park additional vehicles and the road access to get those cars into and out of resort property. Disney's big problem is that too many of its visitors are driving to the parks alone. They're annual passholders dropping in for the day, perhaps meeting other APers at the park. This isn't Orlando's crowd, where entire families fill the majority of cars through the tollbooths.
It's too bad for Disney that Orange County's decided to spend its transportation dollars on expanding freeways instead of building a rail system, as neighboring Los Angeles County is doing. Disney's two other two-park resorts - Disneyland Paris and Tokyo Disneyland - have local transit train stations on property, connecting those resorts to the regions' mass-transit systems. Imagine if those old LA-area Red Cars weren't just an attraction on Buena Vista Street, but if their successors were around today to bring individuals from around Southern California to the Disneyland Resort.
There is a Metrolink line that runs near the Disneyland Resort, but its nearest stop is the Anaheim Stadium parking lot. And those Metrolink trains are scheduled to serve 9-5/Monday-Friday commuters, not tourists. Even if you could get a bus to take you from Disneyland to the station, there'd be no train coming until the next morning if you tried to use it to get home at the end of your Disneyland day.
That leaves Disneyland dependent upon cars and buses to bring people to the resort. But even if Disney spent the money to build additional parking garages to accommodate more cars, it would still face the challenge of getting those cars into and out of the resort via Interstate 5 and Anaheim's surface streets, which are often gridlocked when the parks reach their current capacity. Just imagine how bad they'd be with a third theme park.
If Disney's going to add a third theme park in Anaheim (or a fourth hotel, or an expanded Downtown Disney) it's simply got to find a way to increase the number of guests for every car that parks at the resort.