The answer lies just north of the border
#1: Vladimir Guerrero Jr.
Vlad has been the subject of a lot of rumors this year as the Blue Jays have really struggled to compete in a very tough AL East. Toronto has a lot of money tied up in their pitching and might look to sort of reset until a few of their higher dollar contracts come off the books, and the Mariners should be more than happy to facilitate that.
Guerrero is one of the better first basemen in the league and fits the needs of this team to a tee, Vlad is slashing .292/.362/.473 with 16 home runs and just a 16.1% strike-out rate. His 138 wRC+ is elite and he would instantly be the best Mariners hitter. He isn't a great defender but who really cares at this point? He would add a huge jolt to your lineup and hopefully help kickstart guys like Julio, JP, and Cal.
Acquiring Vlad would likely cost a couple of top prospects, maybe even giving Toronto a long-term solution at first base, Tyler Locklear. Guerrero is under contract through 2025, giving the Mariners two seasons to make a playoff push, which justifies that prospect cost. The Mariners could look to acquire 1.5 years of Vlad in a similar way that they did with Luis Castillo (maybe they could even extend him as well).
Vlad is a true game changer and has shown that is truly one of the best bats in the game. With the recent decision to designate Ty France for assignment, this trade could be closer to the finish line than we think.