I imagine the 'DVD featuring him' is something like a WCW themed release that he's a decent part of, not a solo DVD, but that's just speculation.
Sting keeps signing one year deals with TNA. It's certainly possible WWE could sign him to a short term contract but it's pretty unlikely. If WWE is going to sign a legend they're going to sign them to a 'Legends Contract' that's going to give them rights to use them in video games, action figures and for appearances. I could see them doing that whenever he decides to stop signing with TNA and if they did they'd be crazy not get a match out of him, because if just for one match he'd make a difference. That said - he's unlikely to wrestle Undertaker because Taker is likely only wrestling at Mania and while certain people do seem to think Taker-Sting is a dream match I'm pretty sure WWE doesn't agree, or at least not enough to waste the single match he'll do all year on it.
Plus Sting is ligit shitty in the ring at this point. And Taker is barely holding himself together. It'd be a beyond awful match and they know it. Sting couldn't even have more than a half decent match with Bobby Roode and he's a very good worker.
I could see Sting signing a Legend's contract, coming in, becoming a minor on air character and having a PPV match with some mid-card guy and doing the job. I really can't see them doing more than that. They barely even let Foley do that these days and he's a much bigger star.