Quick thought here, make tier unlocks increase your mist energy bar and charge rage. You hit tier 2 and your mist energy now goes to 150. Tier 3 and it goes to 200. Still recharges in the same amount of time. These numbers were chose at random, could as easily by 125 and 150 or 110/120. It even has some basis in lore: http://wiki.spiralknights.com/Energy