According to a long and revealing interview between MTV and Barry Sonnenfeld, there might be more MIB movies one day, perhaps depending on next month’s box office:
MTV: Wrapping up, you talked about how the ending of this film is “surprising.” As much as you can tease, do you view this film as an ending to the “Men in Black” trilogy? Or are there ways to continue the story?
Sonnenfeld: The fantastic thing about “Men in Black 3″ is that it totally closes out the trilogy, it answers questions that you didn’t even know you should be asking, it leaves you emotionally warm and sad and happy, and it could also reboot the franchise. But I can’t even begin to tell you how to interpret all of that. [Laughs] But I would say this: If your interpretation is that “Men in Black 4″ will only star the worm guys and Frank the pug, you are wrong.
They also discuss how the movie began shooting even though only the first act had been written, and how funny guy Jemaine Clement managed to play the main villain.