I think that with the overwhelming crowds now common almost every day at Disneyland, a 3rd park would be great! It would help disburse the crowds and expand the Disneyland Resort.
In my dream world the 3rd park would have themed lands with unique attractions like Disneyland, but if it had to have lands themed to entire movies (which seems to be Disney's new strategy Ex: A Bug's Land, Cars Land etc.) then I think it would be neat to have a park themed to classic animated movies. Imagine an Aladdin Land, Little Mermaid Land, Sleeping Beauty land, Lion King Land etc. I also think that a Star Wars land would be the ultimate in cool and way more capable of competing against the soon to be Wizarding World of Harry Potter at Universal Studios Hollywood.
Also, in my dream world, the new third park is connected by Monorail to Disneyland, ( I love the monorail and would love to see it expanded). I would also include Disney buying up Garden Walk and building some cool value resorts. The current Disney resorts are beautiful but are out of most people's price ranges in this economy.