The show boss has told how he is planning a radical shake-up next year which is likely to include an extra "novelty" category to take in acts such as Jedward.
And he wants Robbie and possibly another A-list star to join him, Louis Walsh and Dannii Minogue on the judging panel with Cheryl Cole's future on the show in doubt.
Cowell told the News of the World that Robbie - who sang his most famous song Angels last night with Olly Murs and performed his single Bodies earlier in the series - could be the key to making next year's show even better.
He said: "I would like to get Robbie Williams. He's got good instincts.
"He doesn't take himself too seriously and he's a fun guy to be around.
"I'd love to have him there because I think he would be a good judge, full stop.
"That's why I like having Louis Walsh on the panel. He is fun to work with.
"I don't know if Robbie would want to do the show as he's so busy but he would be great. A lot of things are going to change if we bring the show back."
He did not specify what the fifth category would be but Dannii has indicated they are looking at the novelty angle.
Cowell is keen to inject more humour into the show and continue the huge success of acts like Jedward - John and Edward Grimes.
He also denied rumours former Spice Girl Mel B will be taking a seat on the judging panel in 2010 in place of Dannii.
Cowell, 50, said: "We have not approached Mel B to do the show. We met with Mel two or three years ago at the time we brought Cheryl in and didn't know if Cheryl was going to accept.

"We also met Mel earlier this year to possibly get involved with a new show, not the X Factor. She wants to do the X Factor but she hasn't been asked yet.
"No deals have been concluded for next year, nothing. So we're not in a position to talk to anybody about contracts."
The media mogul repeated his threat that if ITV does not find an extra £3million for the show's budget he may AXE the show.
He said the money was necessary to bring in more staff with many of the current team working round-the-clock to put on a good production.
And he stressed he does not want to boost his £7.5million-a-year pay cheque.
Cowell explained: "The reason we haven't announced anything for next year is we're seeing if we can work something out that will allow The X Factor to stay on in the UK for a few more years.
"Everyone is really stretched right now. We need more people working on the show and that costs money.
"There are a lot of people working 18 hours a day, seven days a week trying to keep this show running. We need more people. So I need that commitment from ITV. If you hire more dancers, more stylists, you need to add to the budgets.

"Plus, when we're flying people all around the world we need money.
"Most people out there at the moment are asking for salary increases. We just want the money for the show. I haven't asked for more money. I'm not looking for a pay rise. I wouldn't ask for a raise if it meant we had less money for the show.
"If we don't reach a deal with ITV I will simply take a break. I am not going to work for anyone other than ITV and if we couldn't come to a deal then we would say, 'Thanks for the last few years, we've had a great time and that's it.'
"If we're not going to work with ITV then that would be the end of The X Factor."
Cowell confirmed he will be launching The X Factor America, with himself at the helm, by 2011.
But he vowed to continue fronting his UK shows, which also includes Britain's Got Talent, so long as a deal can be reached. Cowell said: "My gut feeling is X Factor will be on American TV in the next 18 months."
But he admitted his workload has left him so exhausted he wished he could clone himself.
He joked: "I'm going to clone me to do all these shows. We're in the middle of a scientific experiment right now. I'd make 50. I would live with one of them. He'd be like my friend.
"Most importantly I am not running off from Britain to America."
Cowell went on to blame Dannii for ruining Stacey Solomon's chances by trying to make her a clone of 2006 winner Leona Lewis.
He said: "If Stacey were to win the show, we would have to go right back to where she started from when we first saw her.
"I would take her out of those stupid ballgowns and stop her trying to sound like Leona. I think she is a really good singer but all her originality has been stripped away from her.
"Dannii is trying to make her Leona Mark II and you can't do that.When Stacey sang Coldplay she was so good they sent her a platinum disc. She needs to do something that's interesting."
On last night's show, Stacey duetted with crooner Michael Buble on the old standard Feeling Good.
On Olly, his last contestant, Cowell added: "He's like a new Robbie. I spent two days with him this week where we went to his home and I got to know him a bit more. I saw what he was like with a crowd and if he is looked after properly and given some decent records he could do really well."
"But Joe McElderry has been very underrated. He's got a very good recording voice. We're going to take our time on Joe if he does win. We'll do our best and he could surprise a lot of people."
Prince Harry was in the audience for last night's show. The 25-year-old royal is a huge fan and has told pals Joe is the best singer. But he loves cheeky chappy Olly and reckons Stacey Solomon is "hilarious."
An insider said: "Harry's been watching the show since the start. He loves it! He was desperate to see it live and when his aides contacted ITV they were thrilled to get him on the guest list.
"Like everyone, Harry loves Joe and reckons he has the best voice.
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If it means he won't be singing any more, I'm all for it.
By Really.. Posted December 15 2009 at 4:11 PM.