Zach Werenski
Having played his entire career in Columbus, Zach Werenski is an extremely underrated player. A left handed shot, he would be the perfect fit with Chris Tanev on the Leafs top line. He is big - 6'2, 210 - and meaner than his reputation suggests.
Werenski can drive play and score a ton. Like Seider, the only thing preventing him from being a top-ten NHL defenseman is the fact that he plays on a crap team with bad players.
That, and the injuries. Werenski has constantly beein injured and it has prevented him from reaching his full potential.
He has 5 season of 40+ points, and a career high of 57 last season in 70 games. And that's on Columbus. Imagine the point totals with Auston Matthews.
Werenski is 27 and since the Blue Jackets won't likely be any good before he's 30, you have to figure they'd jump at the chance to get some solid assets for him. The interesting thing about Werenski, is that you could probably get a trade like this done without including either Nylander or Marner, just because of his injury risk.
The Leafs have to find good players on bad teams who have the potential to become elite when played in a better situation, and after Seider, no player fits this defenition better than Werenski.