With the Leafs, they need to have enough cap space to re-sign the core four or trade one for maximum value to keep the other three. Next offseason, the contracts of Auston Matthews and William Nylander expire and they will both command raises.
The Islanders GM Lou Lamoriello said they have a problem with having 5 true centers: Matt Barzal, Bo Horvat, Brock Nelson, JG Pageau, and Casey Cizikas. While Cizikas is likely going to retire in the next two years, it makes little sense to have a player like Pageau on their fourth line.
Enter a swap that sends 2015 draft picks back each way. If the Leafs were to target Barzal as a 2nd line center (Tavares said he would be willing to move to the wing), they could see him produce at a very high rate. He is currently the Islanders' best offensive weapon and does so while playing in a very defensive system.