Alright here's my take on it:
The Exo Stranger is a mysterious figure, believed by many to be a Vex-induced hallucination. Both Banshee-44 and Lakshmi-2 seem to be familiar with her. Her weapon is strange in that some parts should not yet exist. Now here's where it turns into a bit of a conspiracy.
I believe the Stranger is Maya Sundaresh, member of the Ishtar Collective team consisting of her, Chioma Esi, Duane-McNiadh, and Dr. Shim. After 'escaping' a Vex simulation, along with some 200 perfect copies of them, they temporarily, with the help of a resident Warmind, put their minds into proxy robotic bodies. They then explored the Citadel with the plan to release their copies into the Vex network and see what they came back with. Maya expresses curiosity in the Citadel, wondering aloud whether you could travel to other times and the like.
Now, this conspiracy is enforced a bit by the similarity between the description of the Deep Stone Crypt and the Citadel. Check out the Ghost Fragment: Legends card for more. I think that Maya entered the Vex network in the proxy body. She traveled through time and space, seeing the Collapse, the City Age, and the eventual victory of the Darkness. Armed with foreknowledge, she hunts through the timelines, looking for the perfect Guardian that can stop the Darkness from rising again and heal the Traveler. It is possible that the one she talks to in the Stranger's Call mission is the future version of your Guardian, involved in some new threat. Is she still connected to the Ishtar Warmind? I have no clue
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Thanks for the input it's nice hearing about lore
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If you like lore, check out the Collective of Knowledge. They'll blow your socks off, especially AnonPig
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No idea what that is but sure
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They're an only slightly insane lore group here one the forums. It's a pretty cool group