There's always a difficult tension with trade rumors, because we'll always probably overvalue our own guys, but at the same time this is an oddly specific rumor so if we take it at face value I think we also have to assume it's because the Mariners probably see greater value in Madrigal than we do. Then it's a question in the other direction, are there guys we think are out of reach who they might be down on for some reason, or other ways the value/perception gap lines up with what the team's needs are.
That said, like I mentioned before I think there's a struggle to find a clean match. Madrigal for a bat doesn't make much sense for the Mariners given what position the Cubs would want the bat to play, so you're left with arms, and most of the potentially fungible arms are probably too good or have too much team control for Madrigal to fetch, even if you're high on his value. That's why part of me wonders if the team would rather cash in Madrigal's value here to not fill a need, but to give resources to make a corresponding move. Whether that's a 3 way deal or simply providing the org depth to feel more comfortable trading existing players, it could take several forms.
Looking at the Mariners roster again though, the one name that might make sense is Gabe Speier. He was a solid reliever when he finally got an extended MLB shot this year, he throws pretty hard for a LHRP(12th of 39 LHRP w/ 50+ IP in FB velo), Fangraphs says he's still optionable this year and he is still pre-arb. He might be available because he's a pop-up success that wasn't *dominant* at age 28 and since he's not set to be part of the Mariners 8th/9th inning, they could see that being a worthwhile swap to fix a lineup spot with a multi-year option in Madrigal.