The difficulty about any Leaf's offseason situation discussion is that what I think the concept of what's necessary to have a good team and what the brass in Toronto believe is necessary deviate almost entirely.
Every team remaining in the playoffs goes at least 5 deep with good defensemen who are either good going both ways or, at the very least, excellent in their own end.
Tampa: Hedman, Bogo, McDonagh, Cernak, Sergachev, Shattenkirk
NYI: Pulock, Pelech, Mayfield, Toews, Leddy
Dallas: Klingberg, Lindell, Heiskanen, Oleksiak, Sekera
Vegas: Theodore, Martinez, Schmidt, McNabb, Whitecloud
With the Leafs, the constant discussion is something alone the lines of "we need a third defenseman", "we just need one guy to bring some size, some grit", "we need one guy to pair with Reilly", or "we just need this UFA defenseman."
I think you need to go at least 5 deep. You need guys who you can just put out on the ice and know "while this guy is on the ice, they're not scoring." You can get that two ways. You can develop it, you can home-grow your talent. That would be like Cernak, Pulock, Pelech, Mayfield, Whitecloud, etc. But Toronto lost interest in the concept of defensive-defensemen long ago. So what's the other option? You can buy them off the market or trade for them. That's not impossible at all. Toronto has the cap space easily to nab up 3-4 defensemen for 3-5 million each, who could sit in the zone and win pucks behind the net, clear the front of the net, get sticks in lanes etc, and only having to sell one or two more pieces. But I don't think Toronto believes there is a need for that. I think the belief in Toronto is still that every defenseman should be this puck-moving offensive juggernaut and a defensive-defenseman at best is "oh we just need one of those."
And really the same thing with the forwards. You hear some Leafs fans talking about Tampa. I think the comparisons are inappropriate at best. But for the sake of discussion, look at what Tampa did over the off-season and into the season before the deadline. They got Blake Coleman for a 1st round pick, and a good prospect. They got Barclay Goodrow for a first round pick. They signed Patrick Maroon, the guy who after winning the Cup with St. Louis last year said "old time hockey's back, screw the speed." They signed Zach Bogosian. And even with all their size and strength and physicality, they're probably still the least physical and ground and pound style team left in the playoffs. Toronto has three lines that are supposed to score plus one line where the old guys reside who were probably only hired because the brass in Toronto felt that there was need for "veteran presence", whatever that means.
The way hockey works, you can never guarantee 100% possession. The other team will have their chances. As a result, teams plan for some lines to be scored on more than they score on their opponents. Because opposing coaches will roll out their best lines against your lower lines. When their best lines are on the ice, they will probably have stronger possession numbers than your lower lines. Unless your lower lines are better than their best lines, virtually impossible unless it's like an international competition Canada vs. some other country, you're going to inevitably surrender more goals on your lower lines than you score. As a result, the measure of a good bottom 6 lines isn't whether those lines can score more on the opposing top lines than they give up, it's whether they can surrender a smaller goal differential to the opposing top lines than the opposing bottom 6 can surrender to your top 6. That's why it's problematic when you have a line 3 that is designed to score. Because even if they score, they will not score more than the lines that they are against. And if they are shoddy defensively, and surrender a significant deficit, while your top lines are going against very defensive bottom 6s of the opposing teams and surrender very small deficits, you won't actually score more on the balance. Together with a very confusing line 4, I think just the vision of the game is very different from how I view hockey, and how I think the meta is developing. And my idea of fixing it would involve significant wholesale replacements, not simply small changes to existing structures.