Hi
talking about MMO's, you usually choose what you wan't to play. The Tank, support and DPS Class.
I feel like Destiny 2 wanted to go that way... a little bit. But at the same time not really. Titans have some shield mechanics but can't really tank any boss or mobs. You just shoot, hide and jump around like all others.
Right now choosing Classes only Change the look, jump mechanic and some abilities.
[spoiler]Don't expect that to come in D2. And not in D3 or D4.[/spoiler]
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Being a certain class shouldn't lock you into any sort of set, and arguably even subclass. Perhaps it could be based on perk tree, and armor perks. Of course, you'd have to balance encounters accordingly, and that isn't something the game in its current state can handle, and I don't know how you would change raids and other activities to fulfill that, without radically changing the game the way is played. However, I do like the idea, because it opens up many diverse playstyles other than "floaty shooty guy"