A crossover megaevent as crisis on infinite earths or marvel secret wars needs a lot of time if you want a really good plot.
Maybe it is a House on Gryphon hill, and this could be a open door for a third module set in Barovia, and later Desert of Desolation, and in the end of this story the PCs would be abducted by the dark powers and sent to the land of the mists.
Now my opinion is the return of the demiplane of dread will be with a "reforged" version of the six modules about the grand conjunction....after something will happen, maybe the dark powers "invading the crystal sphere where the Barovia kingdom in the prime material appears. This means
the children of "monster high" would have got a sandbox with a bigger space to place. Let's imagine a region like the
movie Doomday with infects mutants as demi-werebeasts and savage tribes. Yes, it would sounds like mixing Mad Max, Walking Dead, Doom Eternal, Necromunda and Van Helsing.
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And we could discover Strand von Zarovich was victim of a supernatural conspiracy by supernatural forces to become the founder of the land of the mists, and Ilyanna the bait for the trap, to cause the fall of the knight in the "
dark side of the force". And Azalin was manipulated by Vecna to cause the grand conjunction but somebody has altered this causing a "retcon" of the demiplane of the dread. Lord Soth would come back to Sithicus, but this in its own pocket universe between the crystal sphere of Krynn and the demiplane of the dread, and the god Raistlin from "
that future" as guest artist.
Some darklords would be "killed" to become "vestiges" to cast pact magic with. Other darklords would be so powerful as sorcerer-kings from "Dark Sun".
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What changes would you suggest to the setting?