Filip Forsberg is an excellent first candidate. The versatile Swede has been a favourite of Leafs armchair GM's for several years. He's got the speed and skill to be an ideal winger on either of the top 2 lines. His $6 million cap hit can be double retained down to $1.5M if the Leafs can entice another team to come in and retain a little extra. Forsberg would be a pure rental at a high price, but with 22 points in 21 games this season, he may just be worth the ask.
Tomas Hertl is also in an expiring deal. The 28 year old Czech native can play centre or wing, and we all learned last playoffs how much centre depth can mean to a team. Hertl would be another pure rental in need of double retention, but, like Forsberg, his versatility and scoring prowess could be huge for the Leafs in the post season.
Adrian Kempe is probably the most likely candidate and would be an incredible add for the Leafs. The speedy Swede carries only a $2 million cap hit, and unlike Forsberg and Hertl, he will be an RFA at the end of the season, meaning the Leafs can trade his rights, or try to re-sign him more easily than the other two. He can play either the right or left flank and kills penalties, where his speed makes him a constant threat for short-handed chances. He could be a very underrated candidate should the Kings be willing to part with him.
Dubas likes to be ahead of the game when it comes to adding players before the deadline, so don't be surprised if trade talks heat up well before the trade deadline.