Dante Fabbro
A better option than Carrier, he's just 25 and costs the same, only the Leafs would retain his rights after the season because he's a RFA.
A plus-defender with a little bit of offense to his game, he's way better than Benoit and would be a very nice bottom pairing addition. He even has a bit of upside.
Juuse Saros
This is the dream addition. Of any player who could reasonably be said to be available, this is by far the best one. A superstar franchise goalie signed for one more year at a ridiculously low $5 million cap-hit, Saros is just 28.
The cost to get him would be astronomical, making it unlikely. However, if you could potentially add a franchise-level goalie to Marner and Matthews, I would think you'd do just about anything to make it happen.
Brock Nelson
A historically underrated player, Nelson is 32 and has another year on his contract after this one for six million. Despite being 32, he looks like he's 45 and plays like he's 25.
Nelson has 25 goals this year and is just behind a 40 goal pace. If salary was retained and he didn't cost too much, he would be an excellent addition of depth scoring. I've liked Domi's game since Tavarse was put on the third line, but having 3 x #1 centres should be every team's goal since it makes it so hard to defend against.
According to the Athletic's Player Cards, Nelson is a +8 this year, while Domi is a -2. He's better defensively than Max and he's a 6'4 monster.
I don't really think the Leafs are going to upgrade on Domi, but this is a way they could do it.
Alexander Romanov
A 24 year old defenseman who is signed through next season at $2.5, Romanov is 6'1 215 and he is a great shot-blocker who is a plus-defender.
The Leafs have four defenseman who are bringing positive results to the team - Liljegren, Rielly, Brodie and McCabe - that means they likely need two upgrades on the back-end in order to push Giordano and Benoit into the pressbox.
Romanov would be an excellent third pairing left-side option.