Konecny is carrying a big contract (5.5M$ average) until 2024-25. It's sure going to be difficult for the Leafs to make space for him. Also, their top-6 isn't a problem so I don't see why they would pay a big price to bring him.
In the case of New Jersey, it's a divisional rival, and it's known that GMs aren't too keen to trade within their division, so I guess that the Devils would have to propose a big return for Fletcher to accept trading with them.
This season, Konecny registered 32 points, including 9 goals, in 52 games.