So, I was thinking how to make trading possible without currency and linked to the game time-line story. My idea:
Every account has a timeline of its own, to enter the timeline of a friend you need his timeline key, that he can give to you. Every account can go to other 10 timelines on the beginning. So, you can give keys to 10 friends, after that, you get more invites by cleaning timelines, completing monoliths, every monolith completed gives you one more share, after that, you can trade corruption for other shares, that becomes more and more expensive. So you can potentially end up with around 15 to 20 shares easy shares and infinite more by grinding corruption.
There should be open timelines, that anyone can join to play with random people who can trade items inside this timeline.