Personally, I'm not sold on Polanco, on either side of the ball. That seems like an obvious place ripe for improvement, but I doubt the Twins will look for a SS this winter. Dozier is steady, but range and arm seem to get a little weaker every year. He's not going anywhere in 2018 though. Sano obviously will play third the majority of the time so I don't see much room for improvement there, either. Mauer' good, but he's not going to get better. The OF can't get much better, I don't think. So I think at this point you have to work on the pitching, not the backups, if you want to significantly reduce runs allowed again. I just don't see how you realistically improve the defense much.