The house rules of our campaign
Spellcasting: Spellcasters may cast their daily allotment of spells at no cost. They may exceed those limits at the cost of hit points. Lets try 1hp per spell level and see how that works. That is a 1st level spell costs 1hp, a 2nd level 2hp, etc.
You may also break down your spell allotment. That is, if you have cast all of your 1st level spells you may use a third level slot as 3 firsts, or a first and a second, etc. We will also see how this works.