Twins should keep: Max Kepler (...for now)
This needs to be prefaced with the clarifier that the Twins should keep Max Kepler until the right deal comes around. The priority should be on moving Polanco and finding a package around him that can bring in the starting pitcher the Twins need. With the infield market so slow and shallow, it feels like there's more of a need for what Polanco provides than Kepler.
Minnesota can no doubt find good value for Kepler, but this comes down to potentially finding better value by moving Polanco. There's also a risk of creating a similar situation to what happened last year when the Twins traded Luis Arraez and decided to figure out how to replace him at first base later.
Unlike with Polanco, there isn't a logjam of MLB-ready outfielders to fill the void if Kepler is dealt. The question becomes: who plays right field if the Twins trade Kepler away?
Austin Martin is sort of being plugged into every potential roster hole, from being mentioned as part of that infield group to adding depth behind Byron Buxton in center. He could play right, but without Kepler he'd likely become the everyday man out there. Someone else to consider is Trevor Larnach, who ran the risk of being boxed out with the rise of Matt Wallner, but he might be able to handle the task of replacing Kepler.
That then raises another question: Could the Twins trade Larnach over Kepler?
If Minnesota wants maximum impact and value, Kepler is the best piece to move. However, unless he's being packaged with Polanco in a deal to land a replacement for Sonny Gray, the Twins might be better off riding into the season with him in right and then seeing if they can parlay his success closer to the deadline.