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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Destiny is all about making a whole feeling of magic and the traveler and light. Introducing classes like these would simplify the game's story and overall appeal. Don't add these classes. Also, if these classes were added then you would get killed by one class every time in pvp because of how polar they were. That would be a rage-causing mess