This will never happen. The two biggest AI stars ever are Kelly and Carrie. Kelly won't have anything to do with the show and Carrie hasn't started writing a lot yet. I'm pretty sure that Tamyra Gray wrote the winner's single for season 3 and that didn't work so well now did it. They just need to pay one of best songwriters in the business to write a song. Plus the other problem with writing the winner's single is it's impossible to do because we don't know the final 2, if they are male or female and what kind of music they sing. That's why all the songs have been generic and bland.
What they should do is get someone who writes songs for other Idol shows to write a winner's song. Possibly Canadian Idol or Australian Idol, they have had great winner's singles.