That's just it - the idea of a rare class is that every player can't choose it: it only comes up once in a while.
Just a note on this whole "gating" thing...
I have worked this out with my homebrew game/campaign setting with ability score requirements. It is quite simple, really.
The classes are grouped into categories: Warriors, Mystics, Rogues, Wizards.
"Base/default" classes have their single Primary Ability, a la the original game: Warriors/the Fighter: Strength, Mystics/the Cleric: Wisdom, Rogues/the Thief: Dexterity, Wizards/the Mage: Intelligence.
"Specialist" classes, predominantly ye olde school (and now, again, in 5e) "sub-classes," have a secondary ability and minimum thresholds on both they must meet. Cavaliers must have a decent Strength + Charisma. Illusionists must have a decent Intelligence + Dexterity. ...and so on.
Then, what I call "League" classes (I'm still not 100% on the name, but it's the best I've come up with thus far). To qualify as a League class, the class must:
- Have its own specific Archetype of a class in one of the four core categories.
- They have a specific ability or different mechanic than the default and specialist classes use.
- and (generally) there is a second layer of character option (more akin to 5e's style of "subclasses," or Pathfinder's style of "archetype") that makes them different enough to warrant their own class separate and apart -not 'under"- the default class.
League classes have a Tertiary ability score with minimum threshold they must meet and is, indeed, entirely intended to make these classes the most unusual and rarely encountered types of characters. These are classes that form a "league of their own," hence "League classes."
Ex. League class: Barbarian. 1)They are a cultural archetype of Warriors for specific human peoples, totemic clans of a particular region in my world. 2) Of course, all characters of the Barbarian class have the "Berserker Rage" mechanic/feature...which other warriors, particularly the default, do not have. 3) The Barbarian character, at creation, must choose their Clan: Lion, Stag, Bear, etc.... This clan dictates proficiency with a [exotic] clan weapon, certain non-combat skills, certain features as the barbarian advances, AND their Tertiary Ability. So, to be a Barbarian, you must have a Primary Strength score (it is a Warrior class) higher than the average Fighter or other specialist subclasses thereof, a Secondary minimum score in Constitution, and a Tertiary minimum score (the lowest threshold) determined by the player's choice of clan/totem. (Half-elves or half-orcs of this tribal heritage, raised in a clan, may also be the Barbarian class)
Maybe not he best example, since there is a racial threshold as well, but other league classes allow more species options/are more open (if not universal).
BUT, point being, a Witch's "rarity" could easily be "gated" by adding a secondary and/or tertiary ability score to qualify at character creation.
My own homebrewed Witch ["league"] class uses Intelligence (as an Arcane magic wielder, they are in the Wizard class group), Charisma (force of will and strength of personality is largely important, as well as allowing certain class features to be based on or augmented by Charisma bonus), and
Constitution (there is a degree of "toughness" and ability to "endure" punishment - which may be physical or cause physical damage, inherent to certain types of magic-use). [EDIT: Nope. I lied. Just looked it up and the tertiary ability is determined by the choice of Coven] They have 'Crafts" and "Incantations" as mechanics features that other Wizard types do not have. And must choose their "Coven" at character creation which will determine, among other features, a bonus spell/magic list of Nature Magic, Divine Magic, or more Occult/Supernatural powers (there are several other covens I have thought up, but for ease of entry, and because I run heroic games, I basically limit the initial player to three Covens that cover broad witchy archetypes, fit into the world, and provide the power and tools that would be useful for a life of adventure.