Cesaro is in a situation where has the support of the internet and can go in the ring. However, he's been lost in the shuffle. There are reports of him having a big future, but the King of Swing doesn't have too much going for him right now. If the title is up for grabs, then an interim or short reign with the belt could jump start Cesaro's career that's fallen flat since teaming with Heyman. Also, if WWE does want to do a Cesaro versus Lesnar match, then SummerSlam could be a great platform considering Lesnar was once demanding a title shot.
Kane makes a lot more sense than I want him to, from the three tombstones and the additional beating Daniel Bryan took. It's being worked in that the cause of Daniel Bryan's downfall is the Big Red Machine. Kane and Daniel Bryan's paths are going to have to cross somewhere down the road. In addition to that, people don't want Bryan's reign cut to short. My answer is let him pick up right where he left off. If Kane takes his belt as well as his career. It's another great comeback story for the underdog Daniel Bryan. The only issue is who Kane would feud with until that happens.
John Cena is always a guy you can fall back on and is reliable if you have doubts about the other options. I'm personally hesitant to do this unless Daniel Bryan's going to be away for a long time. I think the money match for WrestleMania XXXI is John Cena vs. Daniel Bryan. For John Cena to become champion, it leaves Daniel Bryan to win the Rumble and battle Cena on the greatest stage of them all. But again, unless Daniel Bryan's on the shelf a long time, I wouldn't want to do this just because I don't want Cena and Bryan's path crossing until WrestleMania. Knowing WWE, they'd pull the trigger to soon and ruin that.
If WWE ever needed CM Punk to return, it's now. The harsh reality of it is that they're down a star player. CM Punk likes being in that main event spot, and having him return (say, at Payback in his home state) could lead to something special. People including myself want a CM Punk versus Daniel Bryan feud with some meaning behind it. Having a bitter CM Punk return to claim the title and go against a returning D-Bry would be all sorts of fun. Though I think at this point, CM Punk's content doing whatever he's doing outside of the WWE...
In the end, it just really stinks that Daniel Bryan's title reign could be cut this short. I just hope it doesn't put a dent in his momentum in the long run. Perhaps a return after the injury will silence some of those idiot fans that have already turned on him. Get well soon, Daniel Bryan!