Well, there is a WoB that says that soulcast corpses can be awakened relatively easily. It would go to figure that soulcast corpses would be useful for many other magical purposes, such as maybe a hemalurgical vessel? I wouldnt be at all surprised if Kel ends up on Roshar and binds himself to a soulcast corpse to use as a vessel, either through hemalurgy, a trick of gemhearts, or some other kind of investiture we havent seen yet.
Also, its important to remember that the Ghostbloods also have a highly (perhaps most highly on Roshar) placed member from Southern Scadrial, home of a particular Soverign if I am remembering correctly. Also, Felt, an era 1 House Venture spy makes an apperance on Roshar as well, proving that the timing for characters from Kel's era showing up in the Stormlight Archive is completly doable, even without being a quasi-mortal cognitive shadow held together by a combo of sheer will power and the final act of a dying shadow of a god like our boy Kelseir.
I have a feeling that at the very least, Kel is going to be a highly influential player in the overarching back story along with Hoid, Frost, Kriss and Nazh, and all the other worldhoppers. Im just sad that its gonna be another 20 years before we get a book starring all of these players at once, if we ever really do.