A trade that meets somewhere in the middle
McNeil for Castillo with some other players attached could work, but we start to ponder too much about the importance of the money exchanged. To offset this, something more in the realm of Castillo for Luisangel Acuna (at least to start) becomes easier to fathom. Acuna, while unproven, gives the Mariners a second base option to immediately plug into the lineup next year. They rid themselves of Castillo’s contract entirely and can use the money elsewhere to improve the club. Maybe they even go out and sign Pete Alonso.
The Mets get a starting pitcher with a reasonable salary. The Mariners lower the payroll (at least temporarily) and add some younger talent. It should take more than just Acuna to get Castillo. The cost shouldn’t be unfathomable, though.
Trade discussions among fans will always lead back to the Mariners because of their lopsided ability to prevent runs and inability to score them. Other Mariners pitchers shouldn’t even be discussed because of their far lower salaries, years of control, and the upside each has. Unless you’d be willing to part with Mark Vientos who has already put together a massively productive year, names like Logan Gilbert, George Kirby, and Bryan Woo are not worth the calories your lips would burn by uttering their names.