When I started on the idea of making schools of magic, I didn’t get it balanced just right. The Magic User 5 talents were neat, but not cohesive, showing a limited range of what a wizard could be. So, I figured magic users could start with a grimore as a base template, or the regular Magic User base template.
To get at what I wanted to do, I had to create a generic “all wizards should be able to do this stuff” base template, moving the variously flavored spell lists to the side. Now all lists of spells relate equally to a strong generic center, instead of having to choose one of several lists of spells as a base template.
Customizing the wizard becomes easier. They can spend a week of study and a talent to gain a spell from any other grimore. Or, they can spend a month of study and buy the grimore as a talent, and then learn any of its 5 spells without further training and also cast them for 3 Awesome Points as unlearned talents from the base template.
This allows wizards to be flexible in their spell choice, as it should be, and also protects their usefulness from level 1 onward. They have to spend time to study, and if they want a bigger pool of “base template” spell talents they must give up a talent. This balances them against clerics, who start with 10 base template spells but cannot expand beyond that easily.