I have been thinking about 'open access' skill learning (to do with the project after the one I am working on) including gaining skills through, for example, spell scrolls or training at a magic guild. One issue is to avoid having several classes of magic user ending up with identical skills. On the other hand, there are some spells that could or should be available to more than one class
Two methods to control spell acquisition occur to me. One is to have (for example) 5 types of magic as separate skill types, one for each class. It could be possible for some classes to be able to learn skills from another type, although not all types. Mages could have the ability to learn Wizard spells, if they can find them, but Necromancers can't do Priest spells and vice versa. The main danger of using that method of control is that, in this example, lucky Wizards and Mages would become very similar. However, it might be possible to limit certain skills to the guild, who would be happy to train one specific class for a specific spell, but not others. It might also make the battle menu more complex and counter-intuitive.
There is also the facility to seal specific skills for a class. This could be used, for example, to stop Mages learning top-level wizard spells, but could get cumbersome if you wanted, for example, to stop Priests learning any Necromancy.
Perhaps a hybrid system tailored to the individual project might be the best solution? More thinking to do, I think.
Edit: More random thoughts...
If you had a consumable item to teach skills/spells, then you might have an issue if it gets consumed if you tried to use it on someone who couldn't learn that spell. It might be better if everyone in the party had a chance to learn it. Come to think, I don't know many documents that auto-destruct as soon as they are read. This suggests either that the event called by the item should try to teach the whole party (succeeding with those eligible to learn the spell), or that the item should not be consumable.
A guild or mysterious old man or woman who lives in a lonely cottage might easily have a preference for one magic using class over another. "I will teach this spell to a decent Druid for 200 copper pieces. I don't really like Mages or Wizards, but if you give me 600 then I would be prepared to do so. Don't ask me to teach it to one of those stuck-up Priests, let alone one of those nasty Necromancers." You would probably want a separate guild or learned teacher for each magic using class.
Maybe spell scrolls found in treasure chests and on skeletons could be used for teaching lower-level spells that aren't particularly distinctive or important. For decent stuff you would have to have the party visit the specialists.
On a different tangent, perhaps a few skills could be learned by leveling, even if most have to be found or bought.