The Leafs have a cup winning core that could be tweaked to win. Honestly they could win it at any time. Add a strong blueliner and they’d be legit contenders. Tavares is coming off the books soon too.
They need more than "a strong blue liner." They most definitely need a goaltender; they definitely need, at minimum, two solid defenseman who have the ability to wear down their opponents and have the ability to make the opposition think twice about heading to the front of the net and they need D-men that actually win one-on-one battles on the boards and in the corners. Because they lack physicality on their blue line, many teams (the Canadiens among them) targeted this in the playoffs and would simply dump into a corner where they know they were either going to physically punish an undersized defenseman or they were going to make that player pull up short to avoid contact. The Canadiens ruthlessly targeted Rasmus Sandin in this way during the playoff matchup between the Canadiens and the Leafs.
What compounds their issues is the problem they have with their defence being way too easy to play against mirrors itself up front. They simply aren't constructed to handle bigger teams that play any kind of physical style. Not only are the Leafs unable to dish out physical play and wear down their opponents over a series, but they also can't withstand it, either. Too many of their important players are obviously adverse to physical play and they just fade away when the games matter.
I have watched an awful lot of that Maple Leafs team with current core and it's painfully obvious they aren't a "tweak" away from anything.
This Leafs team needs to do what that Quebec Nordiques/Colorado Avalanche did in the mid 90's and figure out that it's not enough to have "good" players, but you need to have the right mix of guys and skillsets to win and the Leafs obviously do not.
The Nordiques traded away *three* first overall draft picks in order to re-build the core into a team that could win.
Mats Sundin was dealt to make the team more more difficult to play against and was turned into Claude Lemieux and Sylvian Lefevbre. Eric Lindros was moved in a trade that gave him them a playoff warrior two-way forward in Peter Forsberg, a sorely-needed goaltender in Ron Hextall and an offensive defenseman in Steve Duchene (who was then flipped for more grit and depth in Garth Butcher, Ron Sutter and Bob Bassen) and they moved Owen Nolan for a high end offensive defenseman to feed all that talent up front in Sandis Ozolnish.
These were big, bold, risky transactions to re-shape a roster that they didn't believe could win and turn it into something that could.
If they just sat around "tweaking" all the depth pieces they would have just spun their wheels until the core got too expensive.