Kepler is arguably the most affordable player on this list (and my personal favorite) due to some inconsistencies, as well as being a one-year rental bat. There is no denying his fit with this club though. Kepler has put up at least 2 wins in every season since his full season debut in 2016 (minus the 2020 COVID season where he put up 1.1 fWAR in 48 games).
He fits the CTZ mold to a tee as well with career 18.4% strikeout rate and a 10.1% walk rate. He hits the ball extremely hard (highest average exit velocity of his career at 91.9 mph), he barreled the ball up extremely well (12.2% barrel % was the highest of his career) and his xwOBA (.362) was the highest of his career, and his left-handed power bat would be awesome to see in Seattle (baseball savant shows he would've hit 29 home runs in Seattle in 2023).
The inconsistencies with Kepler look to be gone. He has always hit the ball relatively hard, but like I said his 2023 exit velocity was the highest of his career, and his barrel rate was nearly double his career averages. If Kepler can duplicate that, he could put up similar or even better numbers. He was always up and dwon putting up a 122 wRC+ in 2019, a 109 in 2020, a 97 and 95 in 2021 and 2022, before breaking out with a 124 wRC+ in 2023.
Keplers defense, left-handed power stroke and ability to control the zone are a perfect fit for the Mariners if the cost is right. He's on a one-year deal worth $10 million, so the financial cost as well as acquisitional cost shouldn't be too steep.
I think maybe a Cade Marlowe and a bullpen arm could do it. Or if they would take prospects (I would much prefer this route) then I could see like Jonatan Clase and Emerson Hancock working for both sides.