Well, I feel the likes of Paul McCartney, Queen, Annie Lennox, Elton John and Sting are inevitable. Possibly the Spice Girls?
Seems probable that Adele, Leona Lewis and Take That would make appearances. Perhaps Florence and the Machine too.
I'd imagine that it'll be exclusively British considering we have so many big acts, but should they branch out I could see Kylie being involved and perhaps Gaga fronting Queen.
I think it would be good to represent more (sub)genre's though. Young urban acts like Tinie Tempah, or even dance acts like Calvin Harris that perhaps needn't have a slot to perform on their own but could provide the soundtrack to high energy performances.
It'll be interesting to see what they do. I'm assuming the ceremony will be more of a prolonged entertaining and spectacular performance with music and dance, than the more historical/narrative approach Beijing took. But, then again, we can hype up our inventions and contributions to culture at large too.