New York Yankees
The Athletic's Chris Kirschner recently wrote about the Yankees's offseason plans in this article (subscription required). He stated that part of the Yankees' offseason plans should include a trade for more offense and versatility.
"(Donovan's) OPS would have ranked third on the Yankees behind (Aaron) Judge and Gleyber Torres. The Yankees simply need more good hitters heading into next season, and Donovan fills that hole."- Chris Kirschner
Kirschner also states that the Yankees have plenty of starting pitchers both at their major-league level and throughout the minors. Players such as Michael King, Clarke Schmidt, Randy Vasquez, Clayton Beeter, and Drew Thorpe are all players with high upside, and the Cardinals could even try to acquire Chase Hampton, the team's best-ranked pitching prospect.
Derrick Goold in his chat from this week also discussed the Yankees' interest in a left-handed batter. He stated that the Yankees "are interested in the Cardinals' left-handed hitting or switch-hitting infielders."
Donovan is a talented player with four years of control left, so it will take quite the return from the Yankees to interest the Cardinals. An MLB-ready pitcher along with some prospects would probably be enough to sway the Cardinals into dealing the Gold Glover. It would be tough to see Donovan go, as he is beloved by many fans; nevertheless, he is a valuable player who would bring back multiple strong pitching prospects. It is likely that Brendan Donovan becomes another "the one that got away" player down the road, but the Yankees have plenty of pitchers that would make this deal easier to swallow.