I would like to see a quest line that earns an intergalactic jump ship (hard to get) which then opens up a new galaxy like the Hive home world. Add the portal to the Reef to move to the new galaxy. From there we get 5 or so new planets and/or satellites. I would use known systems or nebulae.
This concept leads to travel to all enemy solar systems.
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That would require bungie to do work