The class system as a mix of "profession" (e.g. fighter, rogue), "lifestyle" (e.g. monk, barbarian), "nature" (e.g. sorcerer), and "faith" (e.g. druid, warlock) is one I find vexing in terms of world-building and justifying PC roles in society and economy. Like, somehow two people training to become fighters, one of whom happens to qualify as a sorcerer by heritage, results in one 2nd-level fighter and one not. It's fine to say, "well, the sorcerer does have to spend time on, you know, sulking over their family curse and learning to invoke and control their blood," but I kinda think they can do that in their off-hours while the other mundane fighter is out carousing or something. I get the class system, but personally don't cotton to it. Woe is me.
Leaning into it, all classes would essentially be a career. Each would have a school, guild, controlling council, or such, however informal, and characters everywhere would identify and be identified as members of that "profession." In some cases it may be overt, whether through class heraldry, tattoos, eye colour, etc. In others cases, inconspicuous but detectable by the right means, like an aura similar to that of alignment or magic type. Multiclassing would be a matter of "approval" by whatever governs the new class, and probably means one can never return to advancing in the original class except under extraordinary circumstances. In such a world, classed characters would be an assumed part of the fabric of society, where someone in a "warlock hat" is no more surprising than a cleric wearing a robe of station, and one might be a well-known rogue on the taxable payroll of an aristocrat. Hmm - a thieves guild union on strike for better benefits. Pretty iffy.
I played with something similar in a 3e homebrew, where every class that featured inherent superhuman/supernatural abilities was a "calling" of the blood, a product of divine heritage similar to the sorcerer description, and a character could only have one of those, ever. (Even a character descended from more than one divinity would only manifest a "calling" of one divinity, likely the one higher in the pantheon.) So one could multiclass a "calling" with a purely skill-based class, say, a fighter/sorcerer or wizard/druid (I treated wizards as skill-based casters), but could not be a sorcerer/druid. I can't say this was entirely successful.
I've found alignment handy as I do like good-versus-evil stories, but I apply it more to gods, NPCs, spells, and items than to PCs. Which is why I have trouble with paladins.
