Stevenson is a young and exciting rookie, and finished this season with 86 tackles, 16 pass deflections, 4 interceptions, and 2 forced fumbles. He was also named as the Bears' rookie of the year.
This trade would never happen, I believe, and the Bears would be making a mistake if they made a trade like this.
Diggs is not the player he used to be, and Stevenson could end up being one of the top defensive backs in the NFL in years to come. He was a top name in just his rookie year, and will likely continue to improve.