Fallon, wouldn't be specifically for the West Coast.
The Tonight Show, has become; especially since Fallon, a worldwide brand that has been well known for many of it's skits and that it's even starting to appear on televised format in external countries outside of the United States.
Plus, and that you have to keep this in mind, Universal Studios Hollywood has been pushing the game (more-so than I'd say Orlando as of now) on making the park, not a random mish mash of attractions and IP's that feels out of place. They are making a genuine effort to make the park more consistent in the theming than any other universal theme park on the domestic side. And with a New York Street being highly rumored, especially as some are saying that SLoP is apparently back from the dead; it isn't a surprise that Fallon is being highly considered, for a multitude of reasons.
I have heard Fallon, and I will stand by on that. And if anything, it will be easier for Universal to bring that; as development cost will be more easier as it would be a pre-existing attraction (outside of constructing a new building, mind you) and that they will be more prepared and ready for the attraction, as they won't be using a shell of a former building in the process.