Louis/Nicole/Mel B/aaaaand Bublé would be dynamite I think! I mixture of madness and knowledge!
I don't think Cheryl should come back! I loved her then, but it would just seem quite desperate for both the show and her. Dannii, naaah. Never saw her as a brilliant judge (A good one, but not brilliant). The panel wouldn't be the same without Louis!

Keep Sharon Osbourne away from it. She knew nothing then, she knows nothing now!
If it was A-List Judges aplenty I'd have: Bublé/Britney/Nicki Minaj/Calvin Harris
Different genres represented, I'd say it could be fun, but it's a mix of current and established. Appeals to the mass market, and I think all four know what makes a star. Nicki's just there for the craziness to be honest.