Well, this is not a new idea but this is how I would like to see the magic system:
To cast a spell it costs mana. The more powerful spell, the more mana it costs. This will give you a tactical choice when to use high costs spells. Spells doesn't need to be memorized before combat. Every spell you know should be availible (either you learn new spells by choice when leveling up or by bying/finding/stealing books). I don't want to go into detail about how fast and how mana should be regained, but some kind of timer is nessecary for this system.
I really want to see a balanced and fun attack/defence magic system. Like a rock paper scissors game. Just spamming fireballs and using you mage as an artillery is not fun. I found some magic battles in BG2 really fun.