One of the more interesting subjects that has crossed my mind is the access to various spells in the 10th Age by wizards-at-large versus the access that specific wizards or organizations have to certain spells and magic. Primarily, spell lists may help differentiate orders or students of certain masters by including some rare spells while excluding other common ones. Customization of access, rather than hard and fast rules about what spells wizards may learn, can give various traditions of magic a flavor without requiring the wizard to take a special kit or subclass.
This idea, of course, comes from the Encyclopaedia Magica series, which lists the frequency of each spell in the D&D world at large. One way to augment these lists is, of course, to create proprietary custom spells which are only available to people within the order, college, or apprenticeship.
The very first question to be asked is usually: "What stops spells from becoming available to everyone in the setting as a matter of blanket effect?" The answer, of course, is that wizards are clannish and anti-scholarly. They hate sharing information, in particular spells, because of the sunk costs involved and the direct effect knowing spells that no one else does has on the power differential between a wizard and his colleagues. Wizards may not be so reluctant to teach commonly known spells, but any that they develop themselves will likely not be available in the world at large until long after their death.
Of course, this brings up a point I've heard in other places, which is that many spells that are widely known by mages are not, in fact, the same spell. I generally have wizards determine what their magic missiles look like individually for this very reason. The logic behind this supposition is that some spells, though they are "common," actually, rules-wise, represent a diversity of common-application spells that are developed in parallel by very many wizards at once. The original magic missile, for example, may not have proliferated at all—perhaps it was merely witnessed by a wizard, who created a parallel copy for his own.
Because of this anti-scholarly attitude, a great many restrictions can be justified. This includes access to spells that create certain magical items, and therefore those magical items themselves. This is one of the ways I differentiated the old Milean Schools—they had not only widely divergent interests, philosophies, and spell lists, but also created vastly different armories of magical items to equip themselves with.
Developing a list of spells known by a wizard's master is an important element of this process. I always have mages upcharge adventuring PCs for learning spells that are otherwise not widely available, as well. Thus, while fireball may cost 1,500 gold to learn, a specialized level three spell that is known only to a handful of wizards may indeed cost upwards of 5,000 gold in training.