The MiB comics sort of based their whole thing about parodying UFO conspiracy stuff and the Men in Black conspiracy theories.

I think a scary movie about them could be cool. But it wouldn't work as a Men in Black reboot. That's kind of like rebooting the "Airplane!" franchise to be about serious and scary airplane disasters.

But the conspiracy theory has been around for awhile. X-Files went into it a lot. But the idea of FBI looking Men in Black or government agents going to UFO witnesses and stuff is kind of a trope that was old before the MiB films came out and turned it into a comedy.