Why Bring Back Wacha?
Of course, he has to actually become a free agent for the Cardinals to have a chance at getting him.
His time in St. Louis may have been marred by injuries, but he has proven that when healthy, he can be a dangerous weapon in any starting rotation. He was always more of a ground ball pitcher with the Cardinals, but he's improving in the strikeout category, and that's something the Cardinals need more of. He also doesn't walk a lot of batters.
He also wouldn't be terribly expensive, making this a move that John Mozeliak and Bill DeWitt might be comfortable making. Organizational familiarity is also a plus, and he would be reuniting with several former teammates.
Granted, the Cardinals need more than just one pitcher, and Wacha isn't someone you would typically mark as an ace. This means that if the Cardinals do target him, they'll need to target other frontline starters.
But what could the possible downside be here, you ask? Well, injuries are still a bit of an issue for Wacha. His shoulder has given him a little trouble this year, and it cost him several months last year, so there would always be that risk if the Cardinals do choose to target him.
A lot of his injury issues occurred in St. Louis, so the Cardinals would have to be sure they understand what they're getting into if they sign him.