The Exo Stranger is the perfect example of a complete throwaway that the community assigns way too much importance to. Her singular purpose in existing was to point us in the direction of the Vex. She accomplished that. She was a plot device and nothing more. She does not need a story beyond what she has. She has enough motivation to have done what she did (with her connections to the FWC as well as possibly being an original member of the Ishtar Collective upon which the FWC is based). She fills isles her purpose and had adequate motivation to fulfill said purpose. She does not need to appear in any greater capacity than she already has (unless Bungie really wants her to) and still allow Destiny to be a complete story.