I am toying with a system that uses the spellcraft skill to determine number of spells you can cast per day, rather than some arbitrary table. Also, it allows for spell DC checks once you have exceeded that number.
Here are the basics:
You can cast a number of spells per day where the total of the levels of spells you cast are equal to your spellcraft skill.
In other words, with a spellcraft skill of 4, you can cast up to 4 1st level spells, or 2 2nd level spells, or 1 3rd and 1st, etc. This is limited that you can still only cast spells that are equal to (spellcaster level +1)/2.
Once you have cast this number of spells, you can continue, but you need to make a skill check vs. spellcraft with the DC being 10 + the level of the spell you are going to cast + the sum of the levels of the spells you have already cast past your max. Failed DC means no spell is cast.
So, if you have spellcraft 4, and you have cast your 4 1st level spells, the next one will be DC 11, the next one DC 12, 13, and so on.
It may seem like a lot of paperwork, but we have fond that you can just tally the spells as you cast them, similar to HP damage, and then it is rather simple.
You can find out more by using the link below.
Belgarath the Ancient One