>>723972210 (OP)
This has been tried multiple times, and the trendline is clear. The more action combat elements you add, the weaker the class distinctions become in reality. Prime examples are tera and Guild Wars 2 at launch.
Tera tried to retain the holy trinity and put more emphasis on timing-based dodging, parrying, ect. Guild Wars 2 tried something similar, but tried to null out the tank-dps-healer trinity, only to discover that setup is a load-bearing component to the class-based MMO's multiplayer endgame content formula.
When distilled down to the very essence, MMO "skill" is precision execution of individual role/class mechanics with a modifier of rote memorization of how to mitigate/defeat encounter mechanics/modifiers. This is fundamentally at odds with Action Combat's improvisation-based essence.
Action Combat in an MMO would be min-maxed to remove all elements that make it interesting, smashing it down to merely an additional set of movement mechanics tied to encounter mechanics.