Edited Oct. 6, 2017 at 10:05 p.m.
They'll only trade him if they know he's not going to re-sign.
But if they did trade - we can start with the closest comparison (in terms of value) from last years rental market = Kevin Shattenkirk. The trade was:
Kevin Shattenkirt
Minor League Goaly (25 years old / undrafted)
FOR
1st (2017)
2nd (2019)
2nd Round Prospect (22 years old / NHL 4th liner currently)
Borderline NHL Forward (28 years old / 4th Rounder Pick)
If the two non NHL players cancel each other out. Were left with:
Shattenkirk
FOR
1st
2nd
2nd
Tavares is more valuable & Montreal can't give up any of their impact players so I bet the Islanders ask for:
1st / Noah Juulsen
1st / Nikita Scherbak / Charles Hudon
2nd / Jacob De La Rose / Artturi Lehkonen
To be honest though none of these guys seem like studs (yet) so I bet they want the picks & are willing to throw in Kulemin + Chimera or some other depth players.
.....But then he probably signs with his hometown Leafs & Montreal ends up having to face Tavares for the next 7 years with a depleted prospect pool.