I’m not so sure I love the idea of Vin and Elend coming back. I mean, at the end of Secret History Brandon talks about the dangerous trope of character resurrection. I agree with him that Kelsier had more of a story, but Vin and Elend? Honestly, it seemed like their story arcs were done.
Alright, here are a few theories, in order of stupidness, lowest to highest:
Spook is still living. Kelsier talks to him at the end of Secret History about working together to “find another string,” or something like that. Wouldn’t it also make sense that Spook gets something out of the deal?
At some point, Marsh becomes a Worldhopper. I’ll go so far as to say that he’ll be seen in places other than Mistborn eras 3 and 4 in the future... I’m excited for that one.
Alright, here’s the crazy one: Lift and Wayne meet somehow, (maybe with Kelsier) and they start a Worldhopping gang. This is the one I’m most excited about. Lift and Wayne, guys. Come on!
EDIT: also, Yolen has/had a base-16 number system (shattering of adonalsium, etc. We’ve heard so much about the importance of the number 16 in Mistborn it seems a waste not to have at least one planet with a base-16 number system, giving more credence to the number.)