The problem is that their sales dropped over time. Thus they had to make a decision. Do the same thing again, and bring out a new edition with a few more units like they have done before (but which hadn't worked this time around), OR make a radical step and create something new.
They decided for the latter, and had to get an idea of how to end things so ...
We know at least that only undead warriors can be Relictors, as Sigmar gathers them exclusively from the Realm of Shyish (death) where Nagash is in charge.
After that, it's purely speculation as to who is who.
I reckon Ionus will turn out to be Settra the Imperishable or Arkhan the Black, but that's just a guess on my part.
@Daft provided the link with all the information, so let me just paste it here (after de-4Channing it a bit)
Sigmar- Grips onto the metal core of the world that was and becomes a true god in the new world. Creates the Stormcast Eternals as a force to oppose chaos.
Tyrion- Diefied in the new world, He joins Sigmar in building the new civilizations. He ...
From the summary at Lustria online I got this information:
Nagash attempts to overpower the forces of Chaos, but even with his monstrous willpower he eventually fails and goes roaring into the abyss and he disintegrates to dust.
A falling boulder falls to crush Allarielle, but for reasons never expressed Malekith pushes her out of the way and his leg is ...
Gotrek is back!
It seems that Gotrek is again a mortal dwarf within the Age of Sigmar in the "Realmslayer" book:
Gotrek Gurnisson was the greatest monster slayer of the age, who met his doom at the End Times. The heroic duardin stepped forth into the Realm of Chaos to fight the daemons gnawing at the world's ending and satisfy his death oath, leaving ...
I checked as many the Age of Sigmar books as I could acquire, and I can’t find any reference to Gotrek or Felix by name.
However, we do have a bit of likely information about Gotrek. At the end of the End Times, as noted in the question, Gotrek took on the role of Grimnir. As such, it’s likely that any references to Grimnir in Age of Sigmar ...