Forwards I would look at are: Tavares, Stastny, Kane, Kovalchuk, Thornton, Rick and Riley Nash, Grabner, Filppula, Derek Ryan, Calvert, Stewart, Vermette
Tavares would be a ridiculous add and the Leafs could boast Tavares-Matthews-Kadri. Stastny would be the perfect replacement for Bozak. Bozak wasnt the greatest skater and Stastny would be similar, but Stastny can provide offense, defensive play, and kill penalties. Thornton would be similar, but I would like him in a lesser role due to his age, skating, and recent knee injuries. Riley Nash, Valterri Filppula, Derek Ryan, or Antoine Vermette would be an ideal 4th line center all win draws, can penalty kill, and are not black holes offensively especially if playing with Johnsson/Kapanen.
On win Kane would provide something the Leafs have not had in a long time. A powerful winger with speed and a deadly shot. Kovalchuk would be similar, just older and not as physical. Nash would compliment the Leafs style with his skating, but bring a physicalness to the team and I believe he would be the perfect replacement for JVR, and at a lesser price tag. Grabner would add to the Leafs speed and that is saying something with how fast they already are. Plus, on a 3rd or 4th line Kadri or somebody could feed him breakaways like crazy. Calvert would be a similar addition to Grabner, just slightly more physical and not as fast, but can skate, can shoot, and if need be can kill penalties. Stewart, I have said for a while, is someone I have wanted the Leafs to have. He brings the physical element that Martin would, but plays right wing and can keep up offensively.
Defense I would look at are: De Haan, Moore, Barbario, and Enstrom.
The pickings on defense via free agency are pretty slim. De Haan is underrated, he wont fix the Leafs woes singlehandedly, but would be a benefit on the back end. I know he has not played on the right side very much, but if either he is willing to switch there or Rielly or Gardiner decide to slide over, it could benefit the team. Moore is a great skater and has some playoff experience with a couple teams I like his skating to replace Polak on that 3rd pair. Barbario is a little known name that I think is a good player. good skater, is left handed but can and has played the right side, and has a bit of an edge to him. Lastly, I like Enstrom, he is getting older and was never known as a physical player, but if the Leafs cant land someone better he is a veteran fallback option that is a good skater and great passer. Could help defending by passing the puck out of the zone better than previous options.