So at the end of The Secret History, Kelsier appears to Spook and declares that "Ignorance almost lost us everything. I'm not going to let that happen again." Then in the end note from Brandon, he says that the story of Kelsier is far from over.
As we know, Kelsier somehow returned to the Physical Realm (likely something to do with the spike in his eye), and then traveled to the South to help the people there with medallions that were presumably invented by himself and Spook. After spreading their new technology, Kelsier disappears.
Now lets consider what Kelsier learned in The Secret History. He knows now that there are other worlds, 16 Shards, traveling between those worlds is possible, and that Hoid is up to no good. He is also immortal, and has the means to share that immortality with others, such as Spook and more importantly Demoux, a known 17th Shard member. Along with immortality, he could share the powers necessary to travel between worlds.
We know that Kelsier has no intention of dying the "final death", and that he continues to meddle in cosmere events. We also know that the way he works is by gathering a "crew" of capable people to work with, and we can assume that he wouldn't be against working with people not from Scadrial. We know that Kelsier has an inflated sense of self-importance (which would explain the crew's presumptuous name). And we know that Kelsier thinks Hoid is a negative force on the cosmere (which gives a motive for the 17th Shard to hunt Hoid). Also, Kelsier having met Nazh gives a reason for Hoid to call them "your friends of the 17th Shard".
Now that its all written down, it does admittedly seem a little flimsy. But thoughts?
P.S. Half the credit for this goes to the user Rosharan