So, in Bands of Mourning, Kelsier somehow seems to get his body back. I had a thought: what if he really comes back at the end of Era 2? Then, (quoting directly from a notebook)
Era 2: Kelsier gets his body back, disappears.
Era 3: Kelsier is a shadowy figure who becomes more and more important as the story goes on. With the closing of this era, the tie holding him to Scadrial is broken, and he can travel the entire Cosmere.
Era 4: Kelsier v Hoid: in space!
What do y'all think?