He just needs to come in and do what he's always done. Bring leadership and some bite, use his big frame and produce as he normally does. Be a good teammate by demanding the best of himself and others. He isn't coming here for offense (specifically), though if he plays on the 4th line, he will produce more than most 4th line centres. He's too proud to just go through the motions.
Marner himself in an interview has acknowledged and accepted that these veterans are coming in to "keep us young players in line". These guys are hungry and they are in T.O where winning a Cup keeps you in the cities lexicon for an eternity. You throw in a young player or two who can jump in from time to time in some back to back games, and you have a more competitive team with players at others ankles pushing them.
Yes, he isn't in his prime, but he isn't 55. He could play until he is 45 or 46. He can still play NHL hockey, he played in a fast league the last 12 games in Europe. As it is, todays athletes who take their craft seriously take care of their bodies and know how to play the game smarter to offset any loss of physical attributes. He has shown his desire to stay in top shape by playing in Europe when he could have just gone fishing. That says enough about how serious he takes this.
Any of the new additions, if they are healthy, could perform better than most of us expect They just need to bring what we expect they will bring and have some pride. Joe is no different.