I think it wouldn't be a bad idea to have the length of auras to be one of the parts that upgrade. For instance, level 1 resistance lasts 1 min but maxed out it could last 2 mins. The cool down would possibly increase as well though from what is it now? 3-4 mins to 5 mins at max.
What I'd really like to see is either an item buff of aura that reduces cool downs. It would probably be easier to balance as a new aura though :}