Lets go ahead and just make them a cosmetic. All ghost shells you’ve unlocked become a cosmetic item you can equip to change the look of your ghost. Lore wise and technicality included, there is no need for having multiple ghost shells that perform one function. We only ever put one on and switch out the mods if we need to.
DO NOT try to make them “more useful”! They’re just fine, just make them an ornament!
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Why not change ghosts so we unlock appearances, but make the actual Ghost a part of our Loadout by basically making it what the Artifacts were in Rise of Iron D1. They could add to our power and stats, grant a passive of some kind and be added into the set bonus system that Bungie is adding.