I'm in the group of people that believe Rock/Cena will be a best-of-three program. I can see a scenario where The Rock comes out to address the crowd and to say good bye. As he's doing this, Cena interrupts him and gets in Rock's face for "leaving the fans again." He does his best to get under the Rock's skin and get The Rock to agree to a re-match. It doesn't work, and The Rock leaves the ring, leaving everyone hanging. Over the course of the next few months, Cena will continue to try and get his re-match with The Rock.
In terms of Lesnar, given the media attention he has received since reportedly agreeing to a contract, I think it'd be a huge disappointment if he didn't appear tonight. If reports are accurate, and the WWE does plan on announcing next year's WrestleMania main event tonight, then I see Brock coming out and challenging The Undertaker. At this point, the only viable threat to the streak is John Cena, and most reports have suggested that they're saving Cena/Taker for WrestleMania 30. If that's true, then they need someone big for next year ... and that the best option for that would be Lesnar.