Adding some depth scoring and some guys that can play up and down the lineup should be key. I'll assume you're going to lose Dermott to Seattle (although that would be disappointing) although Kerfoot and Holl are also possibilities.
I have Nylander playing at 3C, but not permanently. I think he can play there, he is showing toughness and attitude in his play and I bet he'd love a chance to compete there. He can allow the Leafs to get some diversity in scoring lines, while still being able to play offensive zone faceoffs and powerplay time with Matthews or Tavares. I'm struggling to decide if Marner should stay with Matthews or play with Tavares again. I'm going to leave it with Marner/Tavares but could easily entertain a switch. When Nylander comes up Hyman goes down to reunite the ZIP line for defensive duty, but it takes the pressure off of them to create offence (which they suck at). I see the lines looking great like this:
Robertson will get a chance to earn his way up the lineup and play some PP2. If they don't deem he's ready you can easily plunk a Joey Anderson or similar cheap player in there. Robertson may come in mid-season or this may not be the year for him. But I think this lineup allows for more spread out offence while still maintaining the ability to group the big players together. It also gives you a deeper pool of centres which you sorely need.
The two big questions are: Is this enough? (I have no idea, it's the option if you want to avoid some drastic) and are you able to keep Campbell/Rielly the next year (I would hope so).
A few minor trades may happen. I see Engvall and Mikheyev as similar players so to bring in somebody who is capable of creating more offence may have it's benefits.
Defensively I think Sandin deserves a full-time 3rd pair slot with a full training camp and consistent icetime. I would love for Kivihalme or Liljegren to push for the spot beside him, but keeping Hutton as insurance only makes sense.