Over the past couple of years I've had several pc's get whisked away to another plane;

a 5e warlock that wound up in the Far Realms, when he got back was an emotional mess
a 5e goblin shaman landed in a different setting with a human skull rattle named Mr. Bone
a 5e goblin monk wound up in Ravenloft
a 5e cleric wound up in a plane of crystals
and a barbarian from the grasslands of Hyperborea wound up on a small island and lived on fish for years

I want to group them together for a solo run adventure in another plane of existence just because I can.
All stat'd to 3rd edition rules because I don't really play anything else anymore.
I have a few ideas for the plane itself mostly cthulhu-ish.
Suggestions welcome