He defeats Triple H at Extreme Rules and says he's now shown people who the true monster of the WWE is and everyone should now understand that HHH's victory at Wrestlemania was indeed a fluke. He then disappears for a few months. Meanwhile, Cena keeps the title to Summerslam and that's about the time when Lesnar reappears, wanting a title shot for some reason.
Why would Lesnar want a title shot, though? Keep in mind that kayfabe wise, Lesnar was fired in May 2012 and wants nothing to do with WWE, and it's always other factors that keep him coming back (HHH challenging him, attacking Vince for trying to fire Heyman, then HHH challenging him again.) So, I'm thinking Cena gets in Heyman's face over something (even with both Punk and Brock off the show, Heyman can still have a role on the show) and it leads to Cena giving Heyman an AA or something. For revenge, Heyman intentionally brings back Lesnar to win the WWE Championship from John Cena. Assuming Lesnar does indeed that, this could be the cause for the Punk/Lesnar feud in the summer/fall, as mentioned. Punk returns and wants to be WWE Champion again, and while Heyman tries to get them both under control and on the same page, Lesnar just suddenly snaps and attacks Punk when he's had enough. Heyman then tells Punk the weeks after this that if going against Lesnar is the way he wants to go, then it's his choice. He takes the side of Lesnar and tells Punk that going up against he and Brock is the biggest mistake he'll ever make.
I also have my own ideas on how to book the Lesnar/Punk feud but I think I'll save that for a thread of it's own.