Guiding principles of this system:

Mesh Seamlessly with 5e

This means no overlapping rules, such as a class, spell, feat, race, etc with the same name as one in 5e.

This is hard, but it removes ambiguity (are we talking about "your" warlock or "the" warlock?).

Some of the ways I intend to do this:

• Races: there aren't any. The Origins system replaces Race entirely.
• Classes: there are new ones, such as Elementalist, Invoker, Priest, and Witch. Want to play a Sorcerer or Wizard? Be my guest. They're in the PHB.
  • Also, the "classless" classes won't have any overlap (Warrior, Practitioner, Enhanced)
• House Rules: or "Mods" as they may be called, allow piecemeal adoption of alternate rules like Mana
• Skills: not sure how much I'll do here, but technically skills aren't called "Skills" at all in 5e. Food for thought.
• Feats: they're dead, 5e killed them. Move on. Class-based abilities are king (i.e. spells and maneuvers).
• Spells: the entire SRD is off limits. Every class in this system has their own self-contained list with no overlap with each other or the SRD, period.
• Equipment: easy enough, just implement an entire gear system and don't reuse a single name. ;-)

Class approach

Alpha

5e (and System 6) nearly perfected what 3e started with Prestige Classes, but not quite. They haven't quite figured out that every "class" is really the combination of two elements:

• A boring but very important baseline establishing combat style, such as:
  • Lots of hit points, cool figher maneuvers, heavy weapons and armor, etc
  • Loads of mana, powerful spells
  • Cool supernatural abilities
• A distinct flavor that sets the PC apart from others, such as:
  • Death Knight, Blackguard, Bladebound, Psychic Warrior, Spellthief (fighter types)
  • Channeler, Summoner, Manifester, Evoker, Diviner, Specialty Priest (caster types)

The latter tends to have a rich array of very distinct powers, and perhaps a long tail of additional spells or maneuvers to help round out their essential flavor. The former grants a PC much of their foundational power and utility.

5e falls short in a few ways:

• All fighter classes lack the basic tools of their trade--things that used to be feats, or even core maneuvers (e.g. charge, cleave, spring attack, etc), leaving them with extremely boring combat styles. A few paths sprinkle some of this back in, but it's grossly inferior to casters.
• Classes like Barbarian, Bard, Paladin, Ranger get the basic formula right (except as above), but are just not very interesting. An ideal path amps up to 11, and these guys are running at 3 or 4.
• Classes like Druid, Wizard (thanks to paths), and Warlock (despite its horrible implementation) pretty much nail this. I can nitpick implementation details, but they show exactly what a good class system is supposed to do.

Still, while a single class system is useful to simplify character creation for new players, it creates the expectation that flavor comes primarily from class, not path, when the opposite is true (even for classes as vaunted as Wizard). Power and utility definitely come from class, but usually in exactly the same way (e.g. all full casters have virtually the same spell slot progression, all fighters have virtually the same hit/dmg, etc). Path is left holding the bill for virtual all mechanical interestingness, but they are hobbled by lackluster implementation (most notably amongst the fighting classes, where they are sorely needed).

In these rules, we're not endeavoring to rewrite the core classes. That's probably for the best, because if it were up to me, I'd remove most of them and gut the rest. The fighters are soul-crushingly boring, the casters have almost uniformly shitty spell lists, and the hybrids should almost all just be paths of a better class.

How can I fix the classes and paths without rewriting anything? Simple enough: I'll make new "classes", which aren't at all the same ones in the SRD, and all of which have a completely different balance point between class and path power.