"American Idol" is getting closer to assembling a full squad -- the show's zeroing in on finalists to join Katy Perry on the judges' panel.

'Idol' sources tell TMZ ... Lionel Richie's still a favorite to fill one of the 2 remaining seats, and singer Charlie Puth has emerged as a front-runner for the other one. We're told show execs will meet with ABC this week to finalize their choices and make offers.

There could be a country curveball, though ... our sources say Luke Bryan is in the mix because he impressed producers in early discussions. Also, if things don't work out with 2 of those 3 ... we're told Keith Urban is an 'Idol' vet who could slide in as an alternate.

Katy and Ryan are anxiously waiting to see who joins 'em.