I know I know, I'm just being sentimental and trying to bring back something that's lost, but I don't need GW to try and make the Old World live on in a way that would regardless be coherent with AoS (so one can pick any AOS new unit they'd like, for example).
So I was wondering about something, and I ended up reading the last page of Archaon again.
And it occured to me the planet is actually NOT gone (I explain why later).
From there, it's pretty possible to imagine two scenarios:
1) a post End Time events era.
2) and era on the Old World that is concurrent with the AoS timeline.

On point 1)
The world doesn't go in a day, at least the French version makes it abundantly clear (last sentence of the second paragraph, it talks about wars following the opening of the third gate in Middenheim). The chaos gods have all but abandonned it to its fate. When they say the destruction could have lasted a moment or a thousand years, they don't talk about the whole process, but actually when the Oak of Ages is swallowed, that's at least the meaning given in my version of the book (French, remember). The mention of wars makes it clear there's survivors, and not just chaos people either, because they're described as "lost in the end". In that light, it is pretty possible to picture things going on for a while before the inevitable end.
A post-apocalyptic Old World that may last for a thousand years is absolutly not out of question, after all.

On point 2)
At the very end, when the world finally does die out, though Sigmar rides away on the planet's core, the planet itself is apparenty not gone (it's described as cold and barren). From what I can gather, it is probably lost in some gigantic magic storm, or engulfed in the realms of chaos, but it's still there. How did the core get out w/o destroying it, I have no idea. Maybe a bit of the crust has blown up leaving a giant crevasse, maybe it's teleported away (hey, we're in AoS mindset now, remember).
Anyway, the planet itself is still there. And since it's chaos, time doesn't mean much. It's not too much of a stretch, if we look upon what impossible things are achieved in AoS, that someone (Sigmar, an elf, anybody really, why not Slaanesh even? Slaanesh would be the best candidate, I can see nobody else who could bring the planet in an untainted way convincingly - and remember, by the end (s)he didn't really want it destroyed anymore) decides that for some reason they need the Old World back, and manage to pull it out of the Chaos Realm? Possibly in a state that predates the final destruction (the most fun would obviously be during ET, let's say Nagash being back means they can't go further than after he's resurected, but not much further after Glotkin, so we keep some semblance of Dwarf, Bretonnian and Empire. Massive destruction, but possibly recoverable).
Let's say, a "Warhammer - Age of Slaanesh" world, where everybody is given a second chance. Since AoS has probably kicked Slaanesh away anyway, let's have Slaanesh be the only chaotic presence on that New-Old World, giving everybody a second chance, w/o the overwhelming odds stacked for chaos since the three other brothers and Archaon are now gone and happily playing their Realmgate war things.

What do you think?