We need a long cutscenes like Lord of the Rings telling all the story to this point.
We need something like the encyclopedia in Mass Effect BEFORE the Final Shape. It could be a simple kiosk, or something even better like a room in the Tower like a libary, with even a NPC.
It needs to have simple descriptions of enemys factions, and what happened since D1, etc. It needs simple wording and NOT that mysterious language used in the lore books, that you read, and read, and end up understanding nothing.
It could be made of slides, images, cinematics, simple text or narrated by a NPC, it doesn't mater, It needs to be done.
BEFORE THE FINAL SHAPE.
HURRY UP BUNGIE!!!
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Bungie made something around with queen, but it focused more on new player info like subclasses and races compared to actual story milestones.