While you can visit Studio Ghibli, it's not a theme park. It's a museum (hence, the reason it's called the Ghibli Museum). But what we really want is a proper Ghibli amusement park. Like this.
As noted on 2ch, Twitter user Takumi created a map for what he calls "Tokyo Ghibliland". Sure, it's heavily modelled on Tokyo Disneyland, but that's OK. The Cat Bus monorail makes that completely forgivable.