IMHO it's hard to predict who a leader might be in 2018. I believe that a leader has to 'earn his stripes' through longevity and a willingness to put him self in the limelight. As much as I personally dislike this phrase, he has to "talk the talk AND walk the walk". He has to earn the respect of the players and then be willing to assume the mantle of "Leader".
A few from the fairly recent past:
Derek Jeter. As much as I dislike this guy - or rather what I know and see of him - he was the Consummate Yankee. He went on the field, played hard, and took no guff from anyone. And he did it for a long time. He was the face of the Yankees for many years and relished that role.
Jason Varitek. He earned the right to be team leader before this, but the day he stuck the glove in ARod's face he essentially said, "f*** you and f*** the Yankees", and the team would have followed him to Hell and back after that if necessary. That's leadership.
David Ortiz. He's the guy the younger players went to with questions about anything baseball. He was the leader and he knew it. He's the guy who said on television that "This is our f***ing city!" and it struck a chord with everyone. He's the guy who called a team meeting in the dugout between innings and inspired the team to dig a little deeper. (And I know that some believe that can't happen, but anyway....) That's leadership.
I've already posted about Josh Beckett and how I believe he led the team down the right path...and then the wrong one.
Right now I don't see anyone on this Red Sox team who's going to do that but I hope I'm wrong and someone does.