Most of what he shares is specifics about the time travel storyline, which brings Agent Jay (Will Smith) to 1969 in order to save a young Agent Kay (Josh Brolin). And while there, the duo reportedly interact with Andy Warhol, who is of course an alien (other aliens among us include Jimi Hendrix, Yoko Ono and Fidel Castro). Fish-out-of-time jokes apparently include stuff regarding Jay's race, though that seems just a bit out of place as late as '69.
Tassi also claims there is only one villain in the film, a "nightmare biker" type named Yaz, who goes back to kill the young Kay, which somehow will lead to the destruction of Earth. It's assumed this violent character is the one Jemaine Clement is playing, yet back when the Flight of the Conchords star was cast his role was listed as one of multiple villains, he being the "charming and creepy" Bruno.
One thing pointed out that I somehow never thought of is the issue with Brolin's age. The actor is clearly way too old to portray a version of Tommy Lee Jones from 40 years earlier. Or is Agent Kay a lot older than he looks, possibly from some alien age-defying aid? As Tassi says, these movies are too silly to bother thinking about their logic and authenticity. Regardless of their sense, and despite my thinking the first two are only so-so entertainments, I'm kind of excited about this sequel. Given the Back to the Future-like plot, I do hope they get Crispin Glover to reprise his The Doors role as Warhol for this, too.