I decided the PC (a 10th level Monk) of a player who hasn't been with us for a while got hit with Dismissal while guarding away from the group. I decided to use random determination on where he went (from the Fire Node of the Temple of the Elemental Evil, a sort of demiplane off Greyhawk).
Rolled 67 for the 70% chance of going to his home plane, so yeah, Prime Material Plane.
Rolled 5 on the scale of:
1: Flanaess region of Greyhawk setting planet Oerth
2: "Izmerhawk" region of planet Oerth (based on the D&D movies, primarily the 2nd not terrible one, "Wrath of the Dragon God". I did some light developing of this on Facebook's Flanaess Geography group and kind of want to use it.
3: Other weird region of Oerth, like Hepmonaland, Isle of the Ape, or possible New Empyria (from I11) or other areas from Dungeon adventures
4: Another PMP planet with a different "physics" (rule set): Known World with Basic Red Box rules (so only up to 3rd level characters, so Teleport to leave won't work -- maybe from the wizard country?), Nentir Vale with 4e rules, or Golarion with PF1 rules.
5: Forgotten Realms mainstream areas
6: Forgotten Realms weird area (off the original map, so maybe Icewind Dale or Kara-Tur)
So yeah, Forgotten Realms!
I rolled a 54 in the Gray Box, and for the page for Iriaebor. That works. He should be able to learn some things about FR (he's a scholar), probably connect with the ruler Bron, and Iriaebor is in the Lord's Alliance, so he can get to Baldur's Gate or Waterdeep and teleport home from there, I think. His mage NPC (retired PC from a different campaign) associate who runs a teleportation service and has a crystal ball should be able to retrieve him.
Seems like it works for a PC turned NPC to introduce the existence of FR to a Greyhawk campaign.