I'm not sure if any part (or all) of this theory has been posted before, so sorry if it has.
It seems almost certain that, based on Bands of Mourning, Kelsier has a new Physical form.
(BoM Epilogue)
In Oathbringer, the Fused gain Physical form by bonding with Singers during the Everstorm. I think it likely that something similar is going on with Kell and his Physical form, except instead of the Everstorm, Hemalurgy is used.
(Ibid.)
My friend and I recently noticed this WoB:
I realize that a RAFO could just mean that Brandon doesn't want to say either way just yet, but there is the potential (as far as I am aware) that Spook could still be around.
(Mistborn: Secret History Epilogue)
It seems that Kelsier enlisted Spook in his quest to get a new Physical form. If so, it could be that the "new string", at least at times, could be Spook.
My friend and I are still working out some parts of this theory, so if anyone wants to add something that refines, supports, or even contradicts this theory, please do.