Two small but important questions when defining my dnd campaign setting. First, if the Astral sea in the spelljammer setting encompasses all material planes, does that mean that various versions of the same material exist? (Can you spelljam from Toril to another DM’s version of toril in the multiverse?) The second question is, if all of the material planes are in the same astral sea, do they share outer planes? Does each deity have to respond to the shenanigans of thousands of dnd party’s throughout the multiverse? Does one party’s planar shenanigans mess with everyone else’s? Thank you for your clarifications and opinions, these things have been confusing me for a while!

Quote from GreatMageBalasar >>

Can you spelljam from Toril to another DM’s version of toril in the multiverse?

 I believe the answer is no. It is not that kind of "multiverse".

The second question is, if all of the material planes are in the same astral sea, do they share outer planes?

Yes, they do.

Does each deity have to respond to the shenanigans of thousands of dnd party’s throughout the multiverse?

I would say a deity doesn't have to respond to the shenanigans of any mortals.

Does one party’s planar shenanigans mess with everyone else’s?

Are you planning on having a party reorganize the planes or something?

For the first I think that's a "no"; as far as I know D&D doesn't go for things like "mirror universes" in any of the official lore. The closest you get is a few bits of time travel or similar stuff. For the second, that's one of those "it's complicated" answers. For settings like Eberron or Dark Sun that specifically tried to distance themselves from the typical D&D cosmological arrangements they were initially presented as wholly divorced realities, whereas now the material tends to talk about how there's a big cosmic "shell" or similar barrier around the plane that blocks or severely limits the ability of residents of the Outer Planes to interact with that world. As for the scope of deities, generally they mostly seem to be focused on a particular plan, but are some figures who explicitly are prominent on multiple planes- Bahamut as Paladine and Tiamat as Takiss for example were the primary movers of the events of Dragonlance and are also considered the preeminent draconic figures in cross-realm lore.

Really though, all of this has always been fairly fluid and WotC has made a particular push lately to emphasize that any existing lore is just a possible tool/guideline/template not an absolute Way Things Are.

Not really, I was more referring to the events of fortunes wheel or eve of ruin. It seems counterintuitive if there are infinite parties saving the multiverse every week.

Thanks, I was really hoping they wouldn’t touch parallel worlds, that would make spelljammer way too convoluted

more than it already is?

also when I run spelljammer at least, that's totally a thing you can do. I wasn't even aware it was homebrew. thanks

Quote from GreatMageBalasar >>

Two small but important questions when defining my dnd campaign setting. First, if the Astral sea in the spelljammer setting encompasses all material planes, does that mean that various versions of the same material exist? (Can you spelljam from Toril to another DM’s version of toril in the multiverse?) The second question is, if all of the material planes are in the same astral sea, do they share outer planes? Does each deity have to respond to the shenanigans of thousands of dnd party’s throughout the multiverse? Does one party’s planar shenanigans mess with everyone else’s? Thank you for your clarifications and opinions, these things have been confusing me for a while!

 I don't think you can spelljam between alternate Toril's, but you can travel Ethereally from the near/shore of one Toril through the deep ethereal to the shore of another.  You can also possibly mirrorwalk or dreamwalk between such places, or possibly cross from ones underdark into the shadowfell, then from there back into the underdark of an alternate Toril; etc.  You may also be able to reach such alternates by way of the Abyss.  There are several such possibilities such as these to consider, but the gist is two of the same world don't necessarily exist withing the same material planes realmspace.  You need extra-planar travel to reach alternate material planes and the worlds they contain because yes, they do share many outer planes and even some transitive planes.  

I’ve never seen anything to suggest D&D went in for mirror-verse realms in their canon, where are you getting this from? Heck, I haven’t even seen much if anything for a true “space outside of time” as opposed to just variable rates of forward progression.

In canon there are no mirror universes. That said, if you are in a group with multiple DMs and two+ have say FR worlds then spelljamming (or any other method) between them is not a problem. The problem would occur if there were two+ official versions of a world.

Quote from The_Ace_of_Rogues >>

I’ve never seen anything to suggest D&D went in for mirror-verse realms in their canon, where are you getting this from? Heck, I haven’t even seen much if anything for a true “space outside of time” as opposed to just variable rates of forward progression.

 3e is where I retain most of my stuff from.  Manual of the planes discuss the region of dream, the mirrorwalk spell, a temporal energy plane, the deep ethereal, etc.