Split power source/encounter abilities from professions and roles and themes and archetypes. So you can have a party where there’s a divine warrior (aka Paladin), skirmishing archer (aka Ranger), primal warrior (aka Warden) and primal spellcaster (aka Druid) who all take the “Ranger” profession to gain exploration, tracking, survival, herbalism, and etc abilities.
While there is much good in your post, I genuinely think that this is an
extremely unwise move, one that would both weaken the game's design
and make it actually, truly like several of the false smears so frequently attributed to 4e (like "grid-filling" and all classes being "samey.")
The thing is, unlike what a lot of people say, 4e
did not simply add a class for every possible combination of things. Formally speaking, there was never an actual Martial Controller. We got a variant of Ranger that
dabbled in Controller stuff, but no actual, proper Martial Controller. Yet we got two different Martial Strikers (Ranger and Rogue), and two different Arcane Strikers (Warlock and Sorcerer), and two different Divine Leaders (Cleric and Runepriest.) We even got two Shadow classes...and both of them were Strikers (Assassin and Vampire.) It was neither true that they created classes to fill every niche, because several niches remained unfilled, nor that they even ignored a niche if it was already filled, because there were several "duplicate" Source+Role combos. Some, like Ranger/Rogue and Warlock/Sorcerer, were perfectly fine. Other were...less fine and probably could've just been merged together (Cleric/Runepriest.)
When they created a class, they always asked a key question: This is already something we know we need to make. How should it happen?
The example of the Avenger is actually quite illustrative here. Unlike with classes like Warlord and Swordmage, where precedent clearly existed already, the Avenger has no native-to-D&D source material. But that doesn't mean it has no precedent
at all. We can, in fact, see a very clear precedent in things like
Assassin's Creed, or the actual training of ninjas and such: the idea of the silent killer who is utterly dedicated to something outside herself, like a religious creed or feudal lord. That's a worthy space, which is hard to explore with the Rogue as written. It recalls a significant portion of the Van Helsing "vampire hunter" angle that the OG Cleric
used to have, but has slowly lost over time, in much the same way that the Warlord features the "Leader of Men" angle that the OG Fighter used to have but has slowly lost over time. Each boosts that archetype that has been neglected to the level of a proper
class, with its own mechanics, flesh on its bones, so that archetype can really shine.
Further, there were in-lore reasons to have, as someone once phrased it to me, the "legbreakers of the gods": in 4e, gods cannot directly manifest in the world without expending a great deal of power, so they instead rely on giving mortal proxies tiny motes of their divine power that can grow with time. This process is called "Investiture." It
cannot be reversed, except by a ritual which the god's agents must conduct. Once you receive Investiture, that power is
yours to do with as you see fit--which means deities and their churches tend to be very
picky about who they give their power to. Even with being picky though...sometimes, people will go astray. Heresy is a real issue. Hence, you need
somebody who can address that problem. You need someone who can hunt down and capture, or (more likely) kill, the traitorous dogs who would
dare disobey their leaders/god/whatever.
From these two things--the Ezio Auditore swooping zealot-assassin, and the grim internal police hunting down and/or executing apostates and heretics--
directly arise all of the mechanics of the Avenger. It wears only light armor, because that's (more or less) what Ezio wears, and because the whole idea is that you're a sort of priest-assassin, relying on the strength of your faith and your speed and skill to protect you. It wields MASSIVE weapons but with skill and precision, because it is in some sense a
divine executioner. Its "damage" feature is unerring accuracy because
"Thou Sword of Truth, fly swift and sure, that evil die, and good endure." It gets another feature that rewards either ganging up on a target with all your buddies (leading the torches-and-pitchforks mob against the heretics!), chasing down an opponent to slay them yourself (Van Helsing vampire hunter), or enduring the slings and arrows of outrageous opponents and by enduring
ending the target of your wrath.
This isn't some kind of dull checkbox-filling. Each class is, in fact, actually designed to support and fulfill a particular range of fantastical premises. Which is why it's so
frustrating when someone says they want to play a Fighter who does damage but don't want to play a Ranger. "A Fighter who does damage" IS "a Ranger" in 4e terms--that's the class
precision-built to support the class fantasy being asked for. It does, also, include
other class fantasies if they excite you, such as having an animal buddy or being a crazy-prepared wilderness survivalist, but if what you want is to be the ultimate enemy-slaying warrior who can dance a lethal dance of death with your enemies...the Ranger is what will give you that and it will
deliver in spades. (Or would it be blades? Heh.)
The fundamental problem with the "just decouple it!" impulse is that all of the above CANNOT HAPPEN. You instantly, and
permanently, strip out the ability to create unique, tailored, flavorful expressions. By turning the game into a reductive "pick a Source, pick a Role, you're done" setup, necessarily both of those things have to become genericized. Every Defender must,
by definition, work exactly the same way because...they're all literally using the
exact same mechanics. You can't have the crazy differences (and, IMO,
really fun differences) between Paladins who boldly challenge their foes and must never shirk from that challenge, and Swordmages who can screw you over from 20 feet away and then laugh as they
teleport away or
become invisible and now you're screwed no matter who you attack (or even if you don't attack at all!) You can't have the difference between the beefy Barbarian who wades into combat, laughing off the enemy's blows and becoming stronger with every strike she makes, vs the literally lightning-quick Storm Sorcerer who zips around the battlefield, never sitting still.
Under the fully-decoupled design, everything becomes a muddy hodgepodge of everything; every feature must be perfectly generic so it will work with every other combination, and unique and flavorful mechanics disappear. Unless, of course, you re-introduce them....at which point all you've done is create
exactly the same system, just harder to learn and easier to mess up. It's literally "just do it worse, with more steps."
None of this is to say that I don't understand
why people want this. It has a beautiful sound to it: the simplicity! The elegance! Cleanly, neatly supporting everything, eliminating this random list of 25 options so it becomes just picking one option from each of two lists. It has, as I have phrased this quite some time ago,
incredible design meta-aesthetics. But in pursuing that meta-aesthetic, that "beautiful design," you in fact take away something really, really important and get nothing
other than making it pretty and symmetric and slightly shorter.