Acquire at least 1 legit, high leverage bullpen arm.
Going back to the days of the rebuild, the Mariner's bullpen has been a perennial strength of the team. They have developed "quad A" guys, waiver claims, and 30+ year-old relievers to be solid, if not, spectacular, relievers for a young Mariners team. At this point, I think that this has shown that they can develop these borderline roster guys into serious contributors.
Even with the impressive ability to develop bullpen talent, the Mariners need stability. They need a proven guy in the back of their bullpen. This free agency class has some interesting arms, veterans that could be had at a good price, and are solid high-leverage guys. I immediately see Reynaldo Lopez, Joe Jimenez, Matt Moore, and even guys like Ryne Stanek or Robert Stephenson.
Those first 2 guys on that list will probably fetch a 2- or 3-year deal in the $10 million AAV range, while Moore will be on a 1-year deal maybe around $5-$8 million. The other guys are probably $4-$6 million. There is definitely some good value here. While the Mariners will most certainly make their "Justin Topa, Gabe Speier, Trevor Gott" type moves, they would be wise to add another solid to high high-leverage arm.