Mariners send out: RHP Bryan Woo, OF Jarred Kelenic, OF Alberto Rodriguez
Padres send out: OF Juan Soto
This would be the more immediate impact trade. You still get a replacement in Bryan Woo, and you can try and pair him with a vet (Rich Hill?) to limit his innings as he continues to build towards a full seasons worth of work. He looked tired at times in 2023, so expecting 175 innings might not be reasonable, but he definitely has a nice ceiling.
Then it starts to get wierd. Could you send out Kelenic to give the Padres someone to insert right into their lineup? They do have a lot of hitters, but with the depth of their hitting, it could take pressure of Kelenic, and possibly let him start over in a new place (even though I don't think that's a worry yet with him). Kelenic showed the ability to be great on offense to start 2023, and I don't think it's that far of a presumption or prediction to think he turns into a darn good player.
Lastly, you get Alberto Rodriguez who had a strong 2023 between High A and AA. 59 extra base-hits overall shows a great feel at the plate, and a slash line of .300/.381/.504 shows that he was able to hit well all season, not just the occasional blast or drive to the gap. If they decide to go the youth route, Rodriguez has an argument at getting time come 2025, and could be another piece that's close to the bigs if they need it.