Part of the premise of Just The Rules is that specific artifacts — classes, feats, spells, etc. — will initially be jettisoned. The purpose is to get down to the core behavior of the game and build up. This might lead to similar artifacts, different ones, or even the same ones.
For the moment, the game has no classes. I don’t know yet what they will look like, or even if I’ll include them at all.
Existing Class Models
Dungeons & Dragons has always had classes.
BECMI and B/X
Basic Dungeons & Dragons, whether in the Mentzer ‘Basic, Expert, Companion, Master, Immortal’ (BECMI) or Holmes Basic/Expert (B/X) form had a few classes humans could take, and a few ‘racial classes’ used by demihumans (dwarf, elf, and halfling). You could have a single class and took all your levels in that class, and the demihuman classes were limited to a certain level.
In BECMI you could also specialize, after a fashion, on reaching name level. A fighter could become a paladin or an avenger, for example. The demihuman classes stopped gaining ‘levels’ but continued gaining other abilities (for some reason people found it unsatisfying to be stopped at level 8 or 12, when humans could reach level 36… who would have thought?)
Advanced Dungeons & Dragons
Both editions of AD&D would let characters pick race and class independently. Demihumans were restricted in which classes they could pick at all, and even within that they would be limited in their maximum level.
Humans could have only a single class, in principle, but those with adequate ability scores could become dual-classed, stopping progression in one class and starting over in another class. They would keep certain elements from the previous class (hit points!) but using class abilities from the previous class would mean not gaining experience (for the encounter or adventure, I forget which). Once the new class level exceeded the previous class level then abilities from both classes could be used freely, within other limits (no, your Fighter 4/Druid 5 may not use steel platemail)
Demihumans could not dual-class, but could multiclass. That is, pick two or more classes and advance in them simultaneously, splitting the experience points gained between them. Hit points were generated for each class level gained but divided by the number of classes; a fighter/magic-user 4/4 is going to have fewer hit points than a fighter 5.
Dungeons & Dragons 3.x
Race and class are still two separate decisions, and they got rid of the racial limitations! Any race can be any class, with no level limits. Multiclassing, in the AD&D sense, is gone (at least until the gestalt optional rules come in), but anyone can take levels in more than one class — which they now call multiclassing.
Every level, a character can pick a class and gain benefits from that class. This includes Hit Die (different classes have different-size Hit Dice), possibly attack bonus and saving throw bonuses, weapon proficiencies, and other class features.
Unlike dual-classing in AD&D, these don’t overlap: the benefits of both simply add up, within the limitations of each class. A druid who becomes a fighter gains proficiency in heavy armor, but still cannot use steel plate mail.
Also, the rules regarding experience points and level progression really make a difference. A fighter 6 who dual-classes to druid in AD&D starts as a level 1 druid (with 6d10 hit points — you don’t gain new hit points) and needs only a couple thousand or so experience points to reach level 2 druid. By the time the character gains enough that he would have advanced to level 7 fighter, the character is likely a fighter 6/druid 6, or close to it. In D&D 3e, a fighter 6/druid 6 is a level 12 character: 6d10+6d8 hit points, BAB +10, etc… and has spent 66,000 XP to get there, instead of the (translating) 30,000 XP to become a fighter 6 and a druid 6.
The first edition of the Pathfinder Roleplaying Game follows exactly the same model as D&D 3.x, but changed the class definitions significantly to try to balance them and to give each more options (even before bringing in archetypes… comparable to D&D 3.x ‘substitution levels’, and I think better). They also changed the favored class rules; the first version was better than the D&D 3.x version, but as they expanded the mechanism over time I came to dislike it quite a bit. Regardless, class advancement is basically the same.
The Legend system from Rule of Cool (not to be confused with the Runequest-inspired Legend from Mongoose) is clearly based on D&D 3.x but deserves a special mention here because of how they implement classes.
Race and class are still split, and each class is implemented as three tracks, each giving a new ability every three levels. Some tracks might actually have multiple variants (such as the barbarian’s ‘Path of War’ having ‘Rage’ and ‘Dervish’ variants).
You have only a single class, but you can ‘multiclass’ by simply swapping out one of your regular tracks for a track from another class. I found this a brilliant approach, and I admit a very gratifying one because it is similar to how talents work in Echelon.
Dungeons & Dragons 4e
Keeps the race and class divide, but back to single-class advancement. There are feats that can allow some cross-class development, simulating multiclass advancement, but it’s still single class.
Dungeons & Dragons 5e
Basically as D&D 4e.
Adventurer, Conqueror, King
Autarch’s Adventurer, Conqueror, King System is roughly equivalent to AD&D inasmuch as characters each have a single class and demihumans are slightly limited in maximum level. They don’t have multiclasses, though. Instead they have dedicated classes that can only be taken by specific races (only elves can be nightblades, for example).
Otherwise standard single-class advancement, but I thought the addition of race-specific classes was worth mention.
AGE System has either three classes (Fantasy AGE and Dragon AGE) or a single class (Modern AGE). The latter case is basically classless and thus not relevant here.
In Fantasy AGE and Dragon AGE you pick one of three classes (caster, skill monkey, or combatant — names differ between the two games so I’m going off party role). You get only one class and gain certain benefits at various levels in the class. The caster gets better at casting, the skill monkey gets better at skills (and some combat options not available to others), the combatant gets better at fighting and more options for doing so.
The classes are very generic, and a character gains differentiation by gaining talents that give new and approved abilities. At times a character can take specializations, which are a special type of talent that represents more what the character is than what they know. For instance, a combatant could take a talent that makes her better at mounted combat, but ‘knight’ or ‘hussar’ might be specializations representing not just mounted combat, but someone who meets certain other characteristics. A character may choose to gain a talent rather than a specialization.
Where some D&D 3.x-related games allow for multiclassing, in the ‘take levels in different classes’ sense, d20 Modern more or less required it. You started with levels in very generic ‘basic classes’ (basically one for each ability score: ‘strong hero’, ‘fast hero’, etc.), then levels in advanced classes when you qualified. The advanced classes were more capable but more specific: you traded flexibility in choosing your talents for, frankly, better abilities. You could later qualify for prestige classes that were much like advanced classes, but more so.
Shadow of the Demon Lord
Robert Schwalb’s Shadow of the Demon Lord has a different take on it.
The game has only ten character levels, and in those ten levels you gain three ‘paths’. Your basic path is pretty generic (there are four) and you gain five levels in this class (four of which give you new abilities; at level four your race gives you a new ability instead of your class doing so). At level 3 you pick your expert path, and gain expert path abilities at levels 3, 6, and 9. At level 8 you pick your master path, and gain master path abilities at levels 8 and 10.
There are no real prerequisites between paths, though some complement and build on each other better than others. If you want to start as a warrior and then take a casting path at level 3, you are welcome to do so.
Echelon is a classless system, but I think I don’t do that here.
I’m leaning toward the Shadow of the Demon Lord model. I like how the paths are intertwined and don’t have dependencies. I think it will give some wonderful variation in character construction.
If I keep the 20-level range Pathfinder uses, and I probably will, I think I’ll need to either increase the length of each path, or add a fourth path. I’m leaning toward the latter. I’ve noodled it a bit and it looks like the basic path is likely to have seven levels, the expert path five levels, and the master and champion paths four levels each. A character’s basic path is likely to be fairly generic and lay the groundwork for the character’s abilities in general, with the higher paths refining them and improving on them in increasingly specific ways.
Have you though about life path type of systems? Traveller has the most primitive implementation. IIRC Cyberpunk 2020 did something similar. Burning wheel does this also but their system has bugs. At each step of the lifepath you get access to a related set of skills or ability modifiers and lifepaths can lead to other lifepaths. My favourite implementation was the ancient computer RPG Darklands.
I have considered life path systems, and examined a few. Traveller, Human Occupied Landfill, Atomik, Task Force Games… I’ve even noodled creating my own, but as a card-based system, more like a deck builder. I haven’t worked out all the details in my mind, though.