I feel like the combo of Mordenkainen and the Blood War hints pretty strongly for something like Mordenkainen's Guide to the Multiverse, or something similar.
We'll see.
He does keep coming up...
Or he wrote a book "and his whereabouts are currently unaccounted for." Making his appearance in Strahd retroactively like Volo's appearance in ToA.Well, he was in Barovia, which is in the Shadowfell. Perhaps they'll go with the idea of a PC group restoring his memories and then he escapes the demiplane of dread and is now wandering the planes.
So far, signs are pointing away from Adventure material: but in a day we might have a better idea for sure.I think this is probably it. And included will be 4 or 5 short, unlinked adventures similar to YP.
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I also think it's interesting that all of the sudden they're making policy changes/clarifications on the DM's Guild and the timing of that, particularly since it will only apply to newly released material. Almost like they're expecting a new run on the platform. Like say, if a bunch of old campaign settings will finally be opened for use, because of, say, a new release regarding the Multiverse.
I don't think they will dump a bunch of new settings all at once like that, but if it is a guide to the planes book they will be opening up Feywild, Shadowfell, Elemental Chaos, Upper Planes, Sigil, etc.
I don't see how any of those planes aren't already available on DM's Guild, save for maybe Sigil (which is mentioned in core books also, so maybe that's game too?) The extra information may lead to more planar adventures, I guess, but I don't see that as having nearly the same impact on DM's Guild as even just opening up even one of the remaining high-demand settings (such as Greyhawk, Dragonlance, Eberron or Dark Sun), let alone all of them.
I am not saying it would have the impact. I am just saying I don't believe we get a new setting this year unless you count Sigil.