My last campaign took place in the Forgotten Realms. The characters lived in a kingdom, which was threatened to be annihilated by some strange outside force. In the course of the story, the characters found more and more information about what was going on, including the fact that the knowledge they require to save their homeland was long lost. At least it was long lost in the Forgotten Realms. Fortunately, the party found a mysterious gateway that brought them to Golarion. The characters had to overcome some obstacles in this unfamiliar place, including the language barrier, that there was no Weave, clerics missing their gods ... things like that. The players liked it, so after getting the stuff they needed in Golarion, I let them visit another place outside the Forgotten Realms setting before they got home.
That campaign is over now and the crisis was averted. At least for the time being. My current campaign takes place 20 years after the last one. Different player characters, but with some well-known NPCs from the last campaign as supporting cast. This time, the whole universe hangs in the balance! The usual plot, standard D&D fare. Because the players liked the setting-hopping during the last campaign, I plan to include stuff like that in my plot. Last time, I just established that there was a portal to a faraway place and that they had to jump through some hoops to activate it. So the powerful magic that got them to Golarion was already there. They just had to jump-start it.
There are some interesting places they could visit, like Greyhawk, Athas, Eberron, maybe Ravnica. There are no limits. Well … there are some limits. Not only do I have to get them from one setting to the next, I would also like to introduce antagonists who can follow them. That’s why I have to know how powerful magic has to be, in order to transport someone to another setting. There is the planeshift spell, but it only lets you (as the name implies) travel to another plane. So I’m looking for a spell that can do more, some kind of realmshift spell. So my main question is, which level would a spell like that be.
Would it be on the same power level as an 8th level spell, so it could be (theoretically) replicated with a wish, without risking to never cast it again? Would a spell like that be 9th level? Can I wish for it, and potentially lose the power to cast wish again? Or is magic like that beyond 9th level? Maybe Divine Intervention? I’m not sure.
I know that there are some kind of fantasy spaceships available, but I have no experience with Spelljammer. So I don’t know how that works and how powerful a spaceship’s magic(?) engine has to be in order to overcome the boundaries of a setting.