I use archetype in the sense of a perfect example of something. The term is often used to describe the basic four classes of D&D (or the basic 3 + 1 classes if you've been playing since the beginning).
I don't have a lot of standard variant
classes in my campaign. What I mean by standard variant is classes besides the four archetypes (fighter, magic-user, cleric, thief) included in the
milieu. Variants, though present, tend to be one-offs based around a
character concept by a player.
WHAT DO I INCLUDE? My wood elves are based upon the druid class from Eldritch Wizardry. They are one of 2 main classes of elves that survived the cataclysm that formed the campaign world into it's current state. The wood elves adapted and, to a degree, have continued to participate in the world of man.
I've also rearranged the Gnome class to be a bit different from the Dwarves. In my campaign Gnomes favor gems and jewels over gold and precious metals. They also have a special affinity with burrowing animals. Gnomes also breed superior war ponies in The Shattered Lands, prized both by themselves and the Hobbits. Dwarves also seek these fine steeds but IMC Dwarves and Gnomes are in a state of cold war threatening to go hot at any time. Last of all, Gnomes are the weavers of a mysterious silk cloth (garnrillon) prized by makers of magical clothing (e. g. robe of protection).
I included druid-type elves because I felt these would fit in well with my campaign milieu. Gnomes are included for no more reason than they were included in the TLBBs and I wanted them to be more than merely Dwarves Lite.
IS THAT IT? Assassins, martial artists, witches, and similar niche classes can be found as NPCs. While I'm not averse to having a player be a member of those professions, they are not commonly found adventuring and endure a number of prejudices from society.
Assassins in my campaign take the depiction of assassins in fictional accounts of the Far East such as the Amida Tong and ninja. They are Lawful, almost painfully so, and ill-suited to the adventuring life. These men and women are basically living weapons to be used once then discarded with extreme prejudice.
Martial artists, also called brawlers, are adept at weaponless combat in the finest tradition of Welsh and Greek wrestling. No flashy moves or flying kicks here, these guys are deadly close up but mostly useless in a sword fight. Brawlers are the bouncers in most places that need same in the campaign. Many a burly and over-served Fighter has been humbled by these fellows in the bars of my campaign. There is also a sect of monks, The Children of Ashing, who are mainly dedicated to brewing very good beer but have an enforcement arm of staff-wielding brawlers in the vein of Friar Tuck (et al.).
Witches are the healers and folk magicians of the common folk. They have staunch defenders among the people they serve, but they rarely need protection. First, they are dedicated to promoting weal and doing no harm to any living thing. Second, they have a number of magical abilities that make them dangerous opponents.
SO HOW DO YOU ADD CLASSES? I mostly make the player do the work. One wishing to play a variant class IMC must bring me a work up of the class with level progressions, abilities, saving throws, combat ability, and so on clearly laid out. This can be a photocopy of a published class or their own work. I review it, make any changes I feel make the class a better fit for my milieu and hand it back to the player. If he likes the changes, we're good to go. Otherwise, we repeat the process until we're both happy.
I also like players who taking the option of playing into a certain class. Want to be a knight? Play your Fighter in a knightly way. Want to be an alchemist? Have your magic-user delve into the mysteries of alchemy, seeking out masters of that arcane art. I like rewarding campaign level play and gamers choosing this route are rewarded in proportion to their role-playing ability.
THE BOTTOM LINE I will not guarantee a player the chance to use his or her character and the unusual skills of same. If you want to be a Thief-Acrobat? It's up to you to figure out how to make pole vaulting relevant to your group. Why would an Assassin, normally an NPC class, be adventuring with the player-characters? That's up to you the player to decide and then justify, not the referee.