In case I expained it wrongly, you'd start off not knowing any magical runes (the building block of spells). You'd learn them from scrolls etc. There would still be special scrolls out there with runes you'd never seen before which would enable you to build new classes of spells. So you'd maybe need to buy/obtain a strategic 5% of all scrolls to get all the runes you needed, but which 5%?
well, yes, this would be interesting