Wolfgang's design visually conveys a fighter who is big and slow but hits very hard. Wigfrid's design visually conveys a fighter who is leaner, more agile, and hits more often. But of course, Wolfgang hits harder and faster than Wigfrid due to the speed boost he receives in Mighty Form, which allows him to land more hits on an enemy before kiting. Among experienced players for whom maintaining uptime on Wolfgang's Mighty Form is a non-issue, this has made Wolfgang almost unilaterally the fighter of choice once his skill floor is met.
What if the +25% speed boost perk was transferred from Wolfgang to Wigfrid? He would retain his identity as the highest-damage character, and he and Wigfrid would then have slightly more differentiated niches and playstyles as fighters. (Perhaps there would be a conditional activation for the speed on Wigfrid, e.g. she only receives it for a few seconds upon striking an enemy--thus she is forced to use it as a kiting tool rather than as a generic boost with 100% uptime).