I've stumbled upon Ars Magica on wikipedia and, as I don't much care about the setting elements, have been thinking about ways of adapting the magic system to brp.
Now, I haven't read the book, and only know what I've read on wikipedia but would this work:
Take the five techniques (Creo, Intellego, Muto, Perdo, Rego) and the 10 forms (Animal, Auram, Aquam, Corpus, Herbam, Ignem, Imaginem, Mentem, Terram, Vim) and make them separate skills.
To cast a spell a PC would need to succeed in his technique and his form rolls. For example to make a fire cold a spellcaster would need to succeed in both Muto and Ignem rolls (more powerfull spells would require more rolls, I imagine a fireball as "Creo * Ignem * Rego"). The thing is that this would make untrained spellcasters absurdly weak and magic unreliable.
The next thing I came up with is this: One could create a separate spell that had that one use only ,but could be improved separately from the forms and techniques. That would still leave room for improvisation ,but would also grant access to some more reliable magics.
The problems I'm facing are these:
1. I would like to have mastery over forms and techniques improve with regular use of magic, but can't figure out the mechanics for it.
2. How would one use power points to "fuel" spells? I'm thinking about something like "1pp for 1 point of something(be it damage, healing or siz amount of water being summoned) or 3pp for 1d6 points of something"
3. How would this system break when skills start to approach 100% ?
Any and all ideas will be appreciated. Especially if someone who has actually read Ars Magica could comment.