SIMON COWELL has an interesting idea:  A face-off between all of the winners of the various singing competition shows.

--He Tweeted, quote, "Maybe the winner of 'X Factor' this year should compete with winner of 'The Voice' and 'American Idol' in a super final.  Just a thought.  I'd be up for it."  Simon added, quote, "I love the rivalry between all the music shows."

--There's no response from the people behind "The Voice" . . . but "Idol" producer NIGEL LYTHGOE has weighed in, and he doesn't think it'd be much of a challenge.

--He Tweeted, quote, "The problem is the 'Idol' winner will be a star by then."