Outside of Shohei Ohtani, the hot stove is cooking for another Japanese player, Yoshinobu Yamamoto. Having been the Ace of the NPB and 3x consecutive winner of the Sawamura Award, Yamamoto is on the front burner for most teams. The Mariners need Yamamoto for a couple of reasons. A rotation of Castillo, Kirby, Gilbert, Snell, and Yamamoto, of any order going into the season instantly puts the Mariners at the top of the pitching hierarchy of MLB.
The reason the Mariners need both Blake Snell and Yoshinobu Yamamoto is quite evident, If the Mariners signed only Ohtani, the team handicaps itself because of his massive contract. Which could handcuff the team in the future. Signing both aces allows, which in theory would cost the same, if not less of Ohtani's contract, to field a 5 headed monster in the rotation but allows the Mariners to trade Marco Gonzales along with Bryan Woo, Bryce Miller, Walter Ford, and other prospect for needs such as bats.