I’m imagining a classless system which is based on points and skill trees.
Each class can be broken down into 2 or 3 skill trees, and one more tree per subclass.
Each level you gain 10 or 12 points (skill points, or ability points, or learning points, something like that, not sure what the best terminology is).
And then you can go to town investing these points into any ability from any tree you meet the prerequisites for. You should be able to replicate a regular class + subclass exactly if you spend the points in a certain way.
For example, maybe there is a "Martial Tree" which contains weapon masteries at the bottom, and Extra Attack higher up, with the prereq of having some weapon masteries and +3 proficiency bonus. This is a tree which many of the regular classes tap into.
There can be an alternative tree which has Martial Arts at the bottom and also provides Extra Attack higher up.
The fighter might have a Second Wind tree and a Action Surge tree, with abilities higher up which give more uses per day or extra effects or alternative effects when triggering SW or AS (just like the regular fighter abilities).
And then some trees could get "tangled" at higher levels. For example, maybe the fighter’s Second Extra Attack feature has prereqs including: Extra Attack from the Martial tree, Action Surge from the AS tree, and char level 11. And that explains why only the fighter gets this, since they’re the only ones to get Action Surge.
Likewise, the EK’s Arcane Charge can have both Action Surge and some level of magic ability as prereqs, representing another "tanged ability" which requires multiple trees.
It would be hard to balance, but could be very interesting…