More Mage Thoughts

 If a Mage is an intellect-based ability, and channeling energies from external sources, why can't anyone or everyone do it?

In many role-playing games, characters have classes that segregate abilities and powers. Skill-based systems can do this too, if the costs of different skills are such that every character can do some of everything.

My favorite example of this was Asheron's Call. There were no classes, only skills, but the cost to purchase skills varied. Skills like Club or Running were cheap, but Bow and Life Magic or Combat Magic were expensive enough that taking skill choices were meaningful and restricted what the character could be good at. So instead of classes, "builds" developed organically.

I suppose you don't care about any of this if you're just trying to make classes for an rpg to have different classes for gameplay purposes - not necessarily for "balance" ("linear fighters vs. quadratic wizards" and all that). The idea of classes comes from literary stories and tropes; Merlin didn't charge into battle with a lance on a horse, Arthur didn't cast spells. Of course, there are counterexamples of legendary heroes who can do everything but I'm focused on more ensemble situations.

External Power Source: Ritual & Training

Model 1: External powered arcane magic requires extensive intellectual training and the undergoing of a ritual, trial, or ordeal. This training can preclude other options that require significant training, such as sword or longbow mastery.

Magic training comes with an opportunity cost of other skills unmastered. There would likely be rituals throughout the years of training. If there was a capstone ritual to be able to finally cast magic (beyond minor cantrips) only after years of extensive training, the difficulty of the ritual would have significant consequences - if most students fail, there would be a group of people well-versed in the mechanics of magic but unable to cast it at higher strength, and possibly be bitter about it.

Divine, clerical, druidic magic would fit this model but with less academic study.

Internal Power Source: An Ordeal In Itself

Model 2: Internal powered arcane magic does not necessarily require training or ritual but may be enhanced by them (meditation, self-mastery, contemplating nature).

The idea here is that a mage uses their force of will to reshape reality - to break the laws of thermodynamics. But can everyone do that? If not, why not, especially if it doesn't require years of study and training? A world where everyone can start reshaping reality gets very dynamic very quickly.

And that's how you get sorcerers. 

In this model, divine magics might not be divine at all, it would just be the caster's force of will - the priest is the power for the spell, not a deity - though obscured or denied.

Why not both?

It seems the easy way out, but perhaps the most satisfying to me.

Academically trained mages can power spells by themselves to a degree but can channel greater powers from other sources: ley lines, magic crystals, elemental fonts, life force, etc.

Sorcerers, innately magic talented casters are usually the source of their spells.

Divine casters are trained in their various traditions and can power some themselves or channel certain powers.

Back to where I started?

I think what might be more systemically satisfying to me might be overcomplicated for game play.

Not quite fully formed thought, oversimplified: a wizard would use Intelligence for channeling magic power, but also Wisdom (willpower) for fueling without an external source.

To use 5e terminology, material components and arcane foci might be the external source (or link to it), though that seems unsatisfying for minor components.