Actually I think they'd probably get a roster player(to even out cap) like Stepan(who'd be redundant with Matthews) and a good pick. Even with giving up a solid Dman in Chychrun, Matthews is no joke. Leafs would probably have to give up a guy like Dermott back since Yotes will need a D back, but like Stepan being redundant Chychrun would make Dermott redundant, especially with already having Sandin. Stepan would be an upcoming UFA that year which would clear 6.5M I believe.
My idea would be Dermott and Matthews for Chychrun, Hayton, Stepan and a 2021 1st(Would require them NOT to re-sign Hall).
You can then trade Nylander too, maybe package him with that pick to get a top 4 Veteran RD. Maybe go after Risto? Overall a top 4 consisting of Muzzin, Chychrun, Reilly and whoever you get from Nylander would be a huge improvement. They'd take a hit offensively but would have young talent in Laferriere and Hayton on the rise and cap flexibility.
If the Leafs get lucky with the lotto they have a great opportunity to retool their roster.