Any ideas on what the second two gates would have been? I'd have to think that it would be kind of hard to come up with fresh ideas for not just one, but two whole separate parks. Maybe some people who were following Universal's development pre-2001 know?
I would think that when Islands of Adventure was originally planned to be Cartoon World, Universal left a lot of potential themes un-tapped. That essentially left anything non-cartoon related to fill in the extra parks they were supposedly planning on. But Islands of Adventure isn't Cartoon World--it's themed to several different things all being vaguely tied into adventure and/or literature. Those just happen to be about the two broadest themes you could come up with--lots of IP's/theme park-friendly ideas happen to be based on ideas of adventure or literature.
For example, let's say they wanted a jungle-themed third-park (a la Animal Kingdom...makes sense to reflect the competition a little bit). This would have been neat and non-redundant with The Studios and Cartoon World adjacent. However, since Cartoons got the axe in favor of IOA, jungles are repetitive (since IOA has Jurassic Park). They could've also done a more realistic, DisneySea-type park, themed to exaggerated versions of real places--however, with IOA next door, that's now become repetitive due to Port of Entry and (more or less) Lost Continent (which wouldn't have existed in Cartoon World).
I feel like the wide variety of theming found in IOA kind of puts them in a corner. Obviously, the massive resort plans got cut around the time of construction since almost none of the original hotels made it to the final product. Maybe budgets got severely cut during the actual planning period and that's what led to putting everything on the table into IOA? Obviously nobody saw the success of Potter coming, and maybe they thought that whatever concepts for additional parks wouldn't be needed.
In any case, I don't see a third park coming any time soon. There's a lot that needs to be done to their existing parks first. But even if IOA and Universal were absolutely perfect and a third gate was to begin construction tomorrow, I don't know what kind of theme it would possibly implement. Maybe you guys have ideas.