Before I try to present a playable magic system it might be a good idea to talk about some goals.
Firstly, there are three stakeholders involved in the game and each of their interests needs to be protected. There are the people that want to have fun playing non-magical characters, the people that want to have fun playing magical characters, and the GM who wants the first two groups to have fun playing in their game/setting. Okay, some GMs don't give a damn about players but I don't give a damn about those GMs.
1. The Risk of Magic
Magic should not prevent the other players from having fun. In other words, you've got to have Niche Protection (1,2). Jonny Cannon took Guns (Superb) because he wants to be gun-guy, that's his cool. Sally Sneak took Stealth (Superb) because she wants to be stealth-girl, that's her cool. Any magic system has to respect the other characters' coolness. Harry Berlin can't take Magic (Superb) and expect to cast a spell that will give him a superb ranged attack and then cast another spell that will give him superb stealth.
That being said, Harry does want to have the flexibility to compete with gun-guy and stealth-girl, that's his cool. Which leads us to the next goal...
2) The Balance of Magic
Magic should be flexible and it should have the option of being powerful but there has to be limits. What those limits should be involves an interaction between system and setting. A system needs to provide the mechanism for achieving balance and the setting determines at what point that balance is achieved. A poor system would be, spend 1 FP to cast a spell. This may be fine for a low magic normals setting but not for a high magic fantasy setting. A good system should take several things into account:
- the difficulty of magic in the setting
- the power of the spell
- and some price that needs to be paid to balance items 1 and 2
A low power spell in a low magic setting may have a higher cost than a high power spell in a high magic setting but either way a balance must be struck.
3) The Payoff of Magic
Everyone will want to play a magical character for a different reason. Some will just want to blast trolls with Fireballs; some will want to make pacts with dark powers, risking their immortal soul; others may want to wield arcane powers based upon their understanding of the metaphysical properties of the universe. But everyone who plays that character does so for some reason and those needs must be met.
This may not be a complete list but I think it does hit the high points. So everything that follows will be guided by these goals.