I think a Ryback/Lesnar feud could be very plausible, considering the fact that both Lesnar and Punk are connected to Heyman. Lesnar's services could be employed to keep the beast from demolishing Punk. I think it would have the potential to be a very enjoyable feud. It would be like an unstoppable force meeting an immovable object.
Still on the subject of Lesnar, I think Sheamus and himself are well matched. Both are total powerhouses. Huge men with a lot of aggression, a brutal fighting style and short tempers. It would really put Sheamus on a par with Cena or Punk if he could get one over on one of the most dominant superstars in history.
I realise his days in the ring are almost definitely finished, but imagine Goldberg returning for one night to address claims that Ryback is like a new and improved Golberg 2.0. Goldberg stands in the ring in casual attire, explaining how he was better than Ryback is. Then he begins boasting about how he never lost a match fair and square in well over 150 matches. *glass shatters* "THAT'S BECAUSE YOU'VE NEVER TANGLED WITH STONE COLD STEVE AUSTIN"! If that match wouldn't sell tickets, I don't know what would.
The final feud I want to see is Undertaker and Kane, one more time. I realise for this to happen, they'd have to restore some legitimacy to Kane, rather than having him in cheap mid-card comedy acts. Undertaker's reputation is immortalised. I imagine he could arrive out to the ring wearing his wife's best Sunday dress and people would still consider him a badass. Kane doesn't seem as destructive as he used to be, and he's become just another guy in the upper mid card. A feud with the Deadman would put him right back up there with the best.
Are there any particular feuds you'd like to see?