Tavares and Nylander have been great. Once Hyman come back, Galchenyuk gets bumped down and the Leafs have two top end first lines, best in the league.
Personally, I'd actually go about it differently. I don't see Galchenyuk as a great option in the bottom 6. I think he adds plenty of value to the team, but in a scoring role. I also think Hyman truly belongs in the top 6, as much as I love him driving the 3rd line and making Toronto hard to matchup against. I think my preference would be to move Foligno down to 3rd line, upon his return, in a defensive role, where he can a little bit of an offensive punch to that line as well.
Give me these lines when everyone is healthy.
Hyman-Matthews-Marner
Galchenyuk-Tavares-Nylander
Foligno-Kerfoot-Simmonds
Thornton-Spezza-Mikheyev
Nash
Brooks
Engvall
I think that gives you the best overall roster balance, with players playing positions they are best suited for. I think Foligno and Simmonds on a line, with Kerfoot as a relentless little pest in the middle, could be ultra annoying to play against. If you really want to make it a pain in the ass shutdown line, insert Nash and move Kerfoot down to the 4th line.