I don't see how Karl Franz could be the Celestant Prime for two reasons:
1. It seemed pretty clear to me that in the end times, Karl Franz became the God Emperor of Man... umm... incarnation of Sigmar at the point he was cut down. Like a Jedi, he became more powerful than we could possibly imagine. Thus Karl Franz cannot be the CP because he is Sigmar.
2. Even if we go with the soul-replacement theory, the timeline is all wrong. Sigmarines only became a thing during the Age of Chaos. After Sigmar decided to create them, the lore states that heroes are snatched up by him at the point of death. Because Karl Franz died millennia before the Age of Chaos even started, heck, even before the Mortal Realms came into being, he could not have been taken up. Stormcasts had not even been conceived then. Is there any lore suggesting that Sigmar took any souls of long-dead heroes out of Shyish?