Here is the problem with attempting a truly classless system, it cannot exist.
Now, hold your horses, stop that furious typing; ill explain.
The meta idea behind classes is how they function, and what effects they produce. For example, a healer focuses on keeping the team in the fight longer, or a tank soaking up the damage for the team, preventing the softer targets from taking as much damage. Almost everyone who plays any sort of MMO will needlessly already understand this. But i am laying out a groundwork of my explanation here. Secondly, a system of complete customization creates scenarios of the "jack-of-all-trades." Which in a classless system WILL exist as a product of the best possible combinations, these combinations will take time; the more components to a system, the longer it will take to hash out the best combination. But nonetheless a god-tier build will emerge, if anyone else played DDO, you will understand the strength of a warforged-arcane.
This brings me to my point i was originally stating, a truly classless system cannot exist without dooming the game to meta-builds. Which will, inevitably become the dreaded "jack-of-all-trades" builds.
From here, what can we hypothesis.
1. There will need to be restrictions in place to prevent that Tanky/healer/dps build from emerging.
2. Archetypes exist not only for balance reasons, but also as a way to categorize ourselves in the line of combat.
3. Meta will exist whether we like it or not, elitist players will always find the best out of any scenario that offers customization.
4. Sidegrades are ALWAYS upgrades unless they have a tradeoff
Where can we go from here?
Players will always associate their play style into a general archetype, this forms the 6 pointed star of EVERY MMO out there, Three super classes Tank, Healer, DPS. And the 3 Hybrid classes Tanky DPS, Support DPS and Tanky Healer. This playstyle is almost inescapable, even without classes and given a freeform customization, players will find these 6 classes to be the absolute most effective when in parties. They will also find that a jack-of-all-trades is also necessary when running content solo (it will always happen).
Attempting to rewrite this formula will lead us into narrow windows of opportunity where we will have to place severe restrictions on certain builds simply for the sake of "preventing classes", which seems ludicrous when you actually take the time to think about it. What we need to do is not try everything we can to totally shatter any sort of meta, but use it to make the meta classes less defining.
I have seen the analogy to path of exile (which i have extensively played) to suggest that each sub-class not have a specific role, but just something that they have a propensity towards, but does not in any way define a specific build for them. I am absolutely onboard with the idea of a tree SIMILAR to the PoE skill tree. There would obviously need to be some changes to the direct stats that the skill tree there gives, but that could easily allow for a large number of builds.
Consider this, a circle with 6 starting points DPS, Support DPS, Healer, Tanky Healer, Tank, Tanky DPS. Names can be given to each starting areas so as not to directly class each starting area. Each player gets customization points that can start from any of the 6 starting points, from there they bridge out. Allocating nodes will give direct benefits such as increased damage absorption (found near the tanky side of the tree). Throughout this tree is Abilities, these abilities will be the skills that the player will use. For a player to attempt a tanky/healer/dps, they would sacrifice stat upgrades to make the journey to other sides of the tree. For example, a tanky healer starting zone COULD get dps abilities but would require significant investment to do so, BUT will not, and should not stop them from doing so though if that is the intention of their build.
I Like the idea of a limited range of abilities, 3 abilities and an ultimate (preferably on a charge system) that is directly tied with the playstyle of that ability, such that a defense/Tank style ultimate (80% damage reduction for a short time, for example) would charge through the player taking/blocking damage and an offense/DPS ability charging through critical hits or large chain kills. This way the player is rewarded for playing a specific way (this isn't intended to force roles on a player, rather reduce the jack-of-all-trades extreme strength that would result from the access to every ability)
Sounds like i want a classed system, right? Well, yes and no... I don't feel that we should be pigeon-holed into specific classes, but i don't like the ability to make that oppressively overpowered jack-of-all-trades that would result if you simply gave access to any ability. The trick is in balancing the meta classes with the ability to create a completely new class that fits the player.
At the same time, i feel that each player should have 3 "Configurations" that the player could swap to at any time without going back to a safe zone. These configuration changes would (and should) reset the charge on the ultimate and would require, say 10 seconds to reconfigure the frame. Now what do i mean by this? A player could have several setups (indeed far more than 3) but they can only have available at any time three of them. The player can switch their configuration out in the field between these three configurations as needed for a fight or specific situation. The action would cause the player to go into a short "reconfiguration" state which would cause a 5-10 second self stun while the player changes configuration. The player could swap out the available reconfigurations at a safe zone, but not in the field.
To prevent this game from being a simple oneoff without any sort of progression the player could have to fulfill tasks and sink crafting components to unlock more parts of the tree to deepen their customization. For example a task for the tank hex of the tree could be to use a shield to block X amount of damage, then research the upgrade with crafting materials. This is not forcing a tank role on a player, but causing them to play in a specific way if they wish to further delve into a damage absorption side of their playstyle.
I also love the idea of engine limitations where a light engine cannot effectively charge for large amounts of damage, or a heavy engine cannot fly long distances. I love the idea of tradeoffs for specific SIDEGRADES. Never forcing upgrades on a player, since upgrades cause wasted space in a video game.