Alright, so with the strawberry fields purchased and ready to be razed for a new park, I think that an actual theme park should go there - not a water park. It's basically the last chance that the Disneyland Resort will have to build a theme park because it's the only land large enough near the resort to place it on.
However, I would love to have a water park at the resort elsewhere. Where exactly? Well, here's my idea...
Red Box: The Water Park
The Simba lot is the perfect size for a water park. Its area is actually just about the same as the amount of space that Typhoon lagoon and Blizzard Beach occupy. Also, nobody wants the hideous Paradise Pier hotel anyway so this would be a great reason to remove it.
Blue Box: Parking Structure
This would be an addition to the Mickey and Friends Parking Structure. Its size would also serve the demand for parking that a third gate would bring.
Green Box: Downtown Disney Expansion
This expansion would connect the water park to the rest of the resort.
Purple Box: New Hotel
A new hotel that fits the theme of the water park. It would also spread into the water park a little bit and allow direct access into the park by those staying at the hotel.
This way, the resort would get a nice new hotel, more shops and stores, both a new theme park and water park, and parking to fill the demands of the whole resort.