I think if you start talking prototypical 3B, you'd want Bryant there in the long run. He's 6'5", and generally you want tall players on the corners to protect the line. That would give the Cubs 6'5" and 6'3" on the corners with Bryant and Rizzo.
Defensively, middle infield and the corners are extremely different, imo. At 3b, the arm comes into play as it's a totally different timing on the throw. At 2b, I'd think Baez's range would come into play. 3b would limit that, imo.
Generally, a great range infielder can play deep and take deeper angles, which requires a quick accurate throw as well. Generally, you would want your two best infielders with range to at SS and 2b. The double play is the biggest thing here. The best, Cub anyhow I've seen, is Rhino. The MF'er made playing 2b look easy.
Eventually, I see Russell and Baez in the middle for the Cubs. If anyone gets switched to 3b, I would bet on Castro, myself. Then Bryant would go to the outfield.
