I brought this up in the line-up thread, but I think you could kill 3 birds with one stone if you focus on a trade with the Coyotes.
1. Nick Schmaltz - Really talented 3rd line centreman, that can play 2nd line centre with injury or on the wing if needed. Locked in to a contract for another 5 years at a decent number and I think he fits the style in which the Leafs play.
2. Lawson Crouse - Power forward that needs work on his skating, but would fill in the lack of physicality in the line-up. Could play anywhere from 1st - 3rd line wing, but more suited for the 2nd line.
3. Darcy Keumper - Not an elite goaltender, but a really good 1B that the Leafs would love to get for this upcoming season in the final year of his deal.
* add ons based on contracts, value, etc. I'm not sure what the additional players would be, but I feel like if you trade Marner, it has to be in a package deal and not a 1 for 1.
You easily lose the deal in the short term. Marner is a 90 point winger with a bloated contract and mixed reputation within the city. But the important part of the deal is that you spread money throughout the line-up and add 3 spots that you will be looking for anyways. Physicality, depth and goaltending.