Confirmed By: Being one of the four Defenders.
Plot Predictions: Picking things up after Daredevil Season 2, we'll find a Matt Murdock who is still very much reeling from the 'death' of Elektra, as well as having revealed his identity to Karen Page, and being forced to question whether or not he should even be a vigilante anymore.
Of course, he's Daredevil, so despite the inner-turmoil he's still going to be taking to the streets of Hell's Kitchen. It was Matt and Elektra who discovered the literal giant plot hole in Daredevil, and The Defenders should see him investigating that further, perhaps trying to find clues as to what happened to Elektra's body.
In terms of the team, it'll be strange to see Daredevil working with others, but Daredevil Season 2 did build towards that with Frank Castle and Elektra, so he's definitely more open to it. As the biggest hero, it would be a surprise if he didn't fall into a leadership position here as well: he has some knowledge of the Hand, and with Elektra in the mix (and seemingly kicking his ass) it'll be personal for him.
That should feed further into his personal torment, as he comes back into the crimefighting fold only to be met with a former love he thought was dead, and there's a chance he will be the one who has to kill Elektra for good (unless they want her around for Season 3, anyway).