San Diego Padres
There's one primary reason why Cardinal fans should root for the San Diego Padres: Mike Shildt. If he's able to lead the Padres deep into the playoffs in the same year that John Mozeliak and his team toiled in the middle of the pack, it would be the perfect moment for Shildt to give Mo a very specific gesture.
Now, I'm not one who believes Mike Shildt is a transcendent manager who could take any team to the playoffs, but his track record does prove that he's able to manage a professional baseball team well. I don't think Shidlt could have done much better than Marmol did this year with the group of players Marmol was given; the Cardinals had a roster doomed to fail from the start. However, Shildt could have possibly inspired the players and challenged the front office more.
There are no other former Cardinals on the team besides Mike Shildt, but the Padres are a team that has seen significant investment from ownership paired with a wily general manager in AJ Preller. Preller is everything Cardinal fans want John Mozeliak to be now: fast-acting, creative, and proactive.
On the field, the Padres have one of the best bullpens in baseball with Robert Suarez, Tanner Scott, and Jason Adam in the back. They have Luis Arraez, one of the best contact players in the game, Fernando Tatis Jr., Manny Machado, Jurickson Profar, Xander Bogaerts, and Dylan Cease, too.
Essentially, the Padres are everything Cardinals fans wish their team was this year. They traded for an ace (Dylan Cease), they signed a useful outfielder who went off this year (Jurickson Profar), their veterans and future Hall of Famers (Manny Machado and Xander Bogaerts) made contributions this year, and they have a Rookie who is going to challenge Paul Skenes for Rookie of the Year (Jackson Merrill, though Masyn Winn has been nearly as good all year as Merrill). Throw in Mike Shildt and an enviable general manager, and you have a team fans of the Cardinals can get behind.