OK, I have an idea in my head about a magic system where you pay for each spell individually, but the spells are on a communal charge system.
Let me explain, Lets say the mage has 8 charges per day on all the spells. But if he uses three charges to use his Fire Bolt spell, then all the spell now have five charges. If he then casts a Detect Magic, then all have four charges. I am thinking this would be an additional minus one (-1) in addition to any other charge limitations it may have.
SO?????