Its interesting that after a junior career of being a scorer, at the NHL level he has so far been more of a defensively sound playmaker.
For me, the biggest concern with Zadina remains that he needs to get stronger and quicker. He has excellent on-ice IQ, has shown passages of excellent play without the puck, particularly with deft steals and covering pinching d-men, and deffo has an eye for finding his man on the opposite wing. He's still struggling to fight free of attention and keep up with the play when in ramps up.
In the short term, we'd be getting better output from him with Fabbri & Ryan, but I suspect being on the first line is investment in his development. The problem is that Larkin & Brome are not a good fit for him. I don't doubt that Bert for Brome would make an impact (Brome has shown himself to be - as the poster before said - a less defensively effective but more offensively skilled version of latter day Helm - but without the bite that Helm sometimes still brings), as Bert remains great glue for more skilled line-mates, and is great at reading dangerous areas...but I just don't think Larkin and Zadina are a great match. Larkin's motor makes him our best player, but he only really meshes with those who can fit in with him...he's kind of doing his thing regardless, and its up to others to come along for the ride. That's not to say he lacks situational intelligence, but rather that his effectiveness is partly predicated on a playstyle that doesn't work for everyone.
What Mantha has that Zadina doesn't is the size to win a good % of physical battles and fight through coverage and a shot that is powerful as well as accurate and this makes him a danger from more positions. Were they not the same position, I'd actually like to see Mantha & Zadina together as I think they would complement each other well.