Stacking extra attack

[Skrum]

For example Extra Attack should probably be a part of a martial power progression tree that is shared by several martials (like spell slots for casters), so instead of stacking extra attack you would be stacking levels on that tree. Another aspect of improved multi-classing would be reworking the vast majority of spells to have up-casting and more efficient up-casting.

Just as an example the martial tree could be something like:
Level 2: Fighting Style
Level 5: Extra Attack
Level 7: Legendary Resistance (Indomitable makes sense for most martials and it should work like Legendary Resistance, and Fighters should have more weapon specific coolness)
Level 11: Extra Attack (2)
Level 13: Fighting Style (or something else that makes sense)
Level 15: Legendary Resistance (2)
Level 17: Extra Attack (3)
Level 20: Legendary Resistance (3)

Yeah I agree with all this, in a big picture sense. Not sure how I feel about Legendary Resist? But I like the idea of a "martial progression."

I think I would look in the direction of extra attack at 5, 10, and 15 that would progress with any combination of fighter, barb, paladin, ranger, and monk.

Then basically take the battle master's maneuver system and expand it to a 20 level progression. And then if a character wants high level maneuvers, they need to stick to one class (like full casters do). Or they can multiclass a bunch and not get access to higher level maneuvers.