I'm sure there must already be such a game in print?

Call of Cthulhu for once

The Gumshoe system is this, basically.

Or any skill-based system that has careers or professions as templates that highlight the skill areas linked to such.

Barbarians of Lemuria does this with careers that are general catch-all's for the skills.

I think Star Wars 1e could qualify.

Warrior, Rogue, and Mage. One class. Develop the character as you wish.

This is basically the description for any skill based system without classes. OFf the top of my head GURPS would qualify, as would D6 (it has "templates" but those aren't really classes).

Knave is classless, and lets you improve your choice of ability score bonuses (for STR, INT, DEX, etc) as you level up.  Knave hacks such as Glaive and my own Book & Blade also let you choose Feats as you level up for PC customization.  Ability score checks are used instead of skills.

Quote from: Jam The MF on October 30, 2021, 12:55:58 AM
I'm sure there must already be such a game in print?

Escape From Planet Matriarchy! RPG. I have the hardcover. I only use RPG systems that are skill-based.

Savage Worlds does this quite well!

You can build whatever character you want depending on the campaign world your playing in they don't have classes but a list of skills and powers.   I'm in the middle of changing the OS of my world to Savage (currently its PF2).   By the time we get done with the current campaign, I will have my world ready for the Savage conversion as I have already got the races converted over, adding a rune system for magic weapons and armor and a few other tweaks to make it mine

Pretty much half of the RPGs in existence, I think.

RuneQuest was one of the earliest classless responses to D&D.

Quote from: Jam The MF on October 30, 2021, 12:55:58 AM
I'm sure there must already be such a game in print?

Ancient Mysteries & Lost Treasures

https://www.lulu.com/es/es/shop/simon-washbourne/ancient-mysteries-lost-treasures-rpg/paperback/product-1622gjed.html?page=1&pageSize=4

https://www.google.com.mx/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwilgr35_fPzAhUik2oFHZMNBcUQFnoECAMQAQ&url=https%3A%2F%2Fbeyondbeliefgames.webs.com%2FAncient%2520Mysteries_v.1.pdf&usg=AOvVaw0lp7-DahGHhaJaraEhEW_c

Many, many non-d20/OSR games as mentioned.

Within the d20 family, Talislanta for 5e introduces an "adventurer" class as the default for the setting, although it also says you can bring in non-magic-using classes from 5e core if you want.

CoC d20 boiled it down to "offence" and "defence" classes that affected your progression of to-hit bonus and saves. I like a lot of things about CoC d20, the magic system for example, but then I run up against the fact that a middle-aged 10th-level librarian character is much better at fighting than a 20 yr-old 1st-level soldier.

I think that highlights something about how class and level interact. If you just have 1 class and continue to tie combat ability to level progression, then everyone is a fighty-type person at least in an in-game-world sense. You can always rationalise it like the old prof managing to connect a really good punch, but that gets old quickly. In theory the same goes for any ability tied directly to level progression, although combat is the main one usually.

I would probably go with a Fighter class and an Expert class, with the Lamentations of the Flame Princess approach that only fighters get a to-hit bonus progression (er, if I remember rightly - I think this is right at least among the human classes, maybe the demihumans get it too).

Stars Without Number does something similar and throws in a psion class.

Many.

My first choice for this would be BRP (e.g., Chaosium's Basic Roleplaying "gold book", or one of the BRP-based games like Runequest). Depending on what I'm after, I might also consider Rolemaster 2 with a house-rule that all characters use the "no profession" profession, which would provide a universal skill list and skill cost matrix.

That's almost a Zen koan, now that I think about it.  "A game with only one class is really a game with no classes."

In practice of course there's no such thing, if you define character class by its real function in the game, which is to protect character roles by structuring character design and mechanics so that minimum acceptable effectiveness at any one function requires specialization in it.  There's a reason nobody ever tried to play a multiclass fighter/thief/magic-user/cleric in D&D.

QuoteThere's a reason nobody ever tried to play a multiclass fighter/thief/magic-user/cleric in D&D.