We answered this in another thread, but the short answer is mythic exclusive week applies to Legendary Tasks as well.
EDIT: Incorrect - The Mythic from Legendary Tasks will be randomly pulled from ALL Mythics, but will include the new Mythic troop too. Clarification here: Mythic Troops & Legendary Tasks
We may review some hero classes in the future. For now we’d like to review and buff heroes as a whole.