I was thinking a bit about a new challenge and I think it could be pretty fun to have it either be part of "Exhaustion" or a new challenge on it's own:
Unnatural
You have no natural regeneration in this challenge. Only Energy and Rage can be got normally, but HP Regen and Mana Regen are gone. You can only regenerate through spells, food or other items like bandages/potion. Consumes are a must. Reward can either be a title or something like a Repair-Bot, since you have to be in shops all the time anyway.