These 2 sluggers could fill the first 1B issue in the short term
#3: Pete Alonso
Alonso might be the biggest name that gets dealt at this year's trade deadline. The 29-year-old slugger is on an expiring contract and the Mets aren't likely to make a playoff push. Alonso's bat should be a hot commodity.
Alonso is slashing .239/.318/.448 with 19 home runs which contributes to a solid 118 wRC+. For being a slugger, his career 22.1% strikeout rate is above average and would fit nicely into this Mariners lineup that lacks power and strikes out at an extreme rate.
All of that being said, Alonso should not be on the Mariners' radar. The first baseman is a rental and though the prospect cost should be minimal, they aren't a team that focuses on high-profile rentals. Pair that with the fact that there is absolutely zero chance Alonso resigns with the Mariners and Dipoto should explore other options.
#2: Christian Walker
Where Alonso is the most widely known name on this list, Christian Walker is far and away the most underrated. Walker is a key piece in the middle of the Diamondbacks lineup and has a legitimate claim as the best first baseman in the game (despite a crazy All-Star snub).
Like Alonso, Walker is a rental, but unlike Alonso, I think there is actually a shot that the Mariners would resign him. He is 33 years old and though he might require a higher dollar amount, the length shouldn't hinder the Mariners.
Walker is slashing .259/.342/.484 with 22 home runs and solid rates (25.2% K rate and 10.4% walk rate). His 129 wRC+ ranks fifth in all of baseball and he is tied with Bryce Harper and Josh Naylor with 22 home runs. Not to mention, Walker is a Gold Glove level defender.
Walker would be one of the biggest bats this Mariners team could acquire and even though it's a short-term solution, there is a lot better chance they can keep him long-term than someone like Alonso.