Marner for Hanifin. Straight up 1 for 1. No questions asked! Canes get a low tier 2nd line center. Leads get a future elite offensive dman.
If you want someone with the potential to be a center, ask for Nylander. Marner isn't a center, and it'd basically be the Drouin experiment again, but atleast Drouin had a bit more center experience (centered Halifax his last year there, after primarily being a winger)
I also don't think Hanifin has shown enough to be labeled a future #1 offensive defender.
Leafs took Marner over Hanifin in 2015, and I don't think he's done anything to really change their mind. Plus, we are fairly strong in the exact role Hanifin plays with 2 offensively strong LHD with Dermott emerging.