Well, there's the rumor (as stated above) that Sting has signed with the company or is about to do so soon, and will appear on Raw the night after Mania to begin a one year build for a match with Undertaker at Wrestlemania 31. Take that one for what it's worth.
If they want to jump right into Bryan/Batista's rivalry and get Orton's title rematch out of the way, they could have Orton cash in his opportunity for another shot at the title in the main event of Raw instead of at Extreme Rules. Having Bryan's first title defense headline the post-Mania Raw would be a cool idea, and the match could be billed as the final chapter in the Daniel Bryan/Randy Orton saga. And at least we wouldn't have to suffer through another PPV with these two facing off with each other again. (More than likely, we will, though.)
I don't watch NXT and thus aren't aware on who could be getting groomed to debut soon, but I wouldn't be surprised to see one or more new faces brought up to the roster. The period after Wrestlemania always feels like it's technically the start of the new year for WWE since Wrestlemania is what everything is building towards all year round, and they often hold off on debuting new blood to the roster until after Mania to give a fresh feel. No idea on who it could be, though.