Juan Soto
The most significant move of the offseason was Juan Soto's decision to choose the Mets' money over the Yankees' historical dominance. Soto is one of baseball's best hitters; he won't lose his skills because he's playing in Queens, but fantasy managers should taper their expectations for him.
First, he leaves the friendly confines of Yankee Stadium for the vast expanses of Citi Field. Yankee Stadium has the shortest right field wall in the majors, and Citi Field was the 28th-best park for left-handed hitters last year. It's unlikely that Soto will be able to hit as many home runs as he did with the Yankees for the Mets.
Secondly, when he was on the Yankees, he had the best hitter in baseball, Aaron Judge, hitting behind him. Now, he has Mark Vientos.
Vientos had a nice postseason and seems like a promising young player, but his OPS was over .300 points lower than Judge's last year. That means fewer runs scored for Soto. And while Fransisco Lindor should set Soto up with many RBI opportunities, the bottom of the Mets lineup is stuffed with guys who don't get on base or hit for power, so he won't see a huge uptick in his run production.
If you play in an OBP-centric league, Soto will remain a top-half-of-the-first-round consideration. But, for people in classical AVG leagues, he might be relegated to the end of the first or early second round since he won't be stealing bases, and his counting stats will suffer playing with an inferior lineup him.
It's fair to wonder if we'll look back on his decision 15 years from now and wonder how his place in the record books — both in real life and in fantasy baseball — was affected by his choice to jump ship to New York's "other" team.