The whole Stromcast Eternal thing doesn't start for many thousands of years after The End Times. As it stands we don't know who any of them are, BL has not explained that yet.
They've pretty much said that any hero or villain dead or alive at any point throughout Warhammer Fantasy could potentially be a Stormcast Eternal, as Sigmar Heldenhammer gathered the strongest souls he could from thoughout time after The End Times to build his new army.
What happens to Karl Franz in Altdorf is:
The Lord of The End Times spoiler alert
his body is actually possessed by a shard of Sigmar Heldenhammer. Sigmar binds the Wind of Heaven to Franz' body, and in doing so possesses his dead body and turns him into an incarnate. This becomes clear and is explained in The Lord of The End Times.