Are there parallel or alternate universes in Magic? Such as… - Ravnica-2 where each guild is a 3-color combination - Theros-3 where gods are artifacts - Jace (from Vryn-5) and Jace (from Vryn-7) meet each otherPlanes can have alternate universes, yes, but it's not common. The three planes we know have them, or
had them, are Dominaria, Rabiah, and Tarkir.
Dominaria's you can see in the set Planar Chaos, there's one where Mirri strikes down Selenia instead of Crovax, one where the Odyssey legends are color shifted, one with different primeval dragons with the wedge colors instead of shards, one where Serra was obsessed with Sphinxes instead of angels, that kind of thing. It's possible these are all the same AU, I'm only listing them as different, we don't have more stories about them than the basics. In the Planar Chaos novel we also see "Ice Age Phyrexians" but don't learn more about them.
Rabiah is refracted 1,001 times, which in the diegetic reason there are no legends in the set. Each refraction is different in some way, although it's not really explored much except in Taysir's backstory for the five versions of him that fused.
Tarkir is more recent, but we've see the Khans timeline, which doesn't exist anymore.
morfurnitureforvess:
Could pre-Alara Reborn Alara fit in that too, since each shard seemed like it was an alternate version of the plane (minus the appropriate missing colors). Maybe I misunderstood but it seemed like when the plane became whole, each of the pieces overlaid where they corresponded with each other (and even had similar architecture and landmarks in some cases).
Alara was a single plane that was split into five, and then those five shards all developed independently without access to two colors of mana for centuries/millennia (it's not clear). They were more like Kaldheim's realms than AUs.