There's been talk about what a roster would look like if none of the big-3 took discounts to re-sign here. @billius27 threw together a potential line-up to show what that could look like:
I'd put Marner's number at $9.5 and Nylander's at $8mil, but it still works out the same. Horton's been LTIR'd, Kessel's retention is there, and it assumes no overages from this current season.
Obviously, the roster is pretty darn top heavy, but that was going to be the case regardless of what happens. The big loss here is Gardiner, but there really wouldn't be any chance of re-signing him without other moves even if Nylander comes in $1-1.5mil less than that and Matthews/Marner's numbers are a smidge lower.
Really though the contract that is mucking up the works here is Marleau's. If the Leafs are able to do something about that (and the structure of his deal makes that possible, but it'll be completely up to Marleau) then they'll definitely have room to either try and get Gardiner re-signed or improve the defence in other ways. If Marleau decides to play out his 3rd year here, then we're forced to go with a budget defence for 19/20 and things will open up a little bit after that.