My idea would be a mini sevitor or hydra skin so you could hold a little one in your hand (i might pay money for this... BUNGIE)
Post your ideas and links below.
Edit: different ghosts color or shape
Edit: there are a lot of great ideas here keep it up. Now if only I could actually happen Ingame... /:
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After you kill a raid / strike / story mode boss, there'd be a chance for a skin to drop to mimic that boss I.e. Beat atheon, the skin would make the ghost crystalline and see through. Beat crota, the skin would turn the ghost Croats color and it would have three eyes.