The instalment imagines what would have happened if Meredith's mother Ellis, played by returning guest star Kate Burton, had not passed away.
Speaking to TV Line, Rhimes said: "To me, the most interesting way of viewing an alternate universe is, 'What if Meredith had never been dark and twisty? What if she's a bright shiny girl?'
"Well, for starters, she never would've been in a bar to meet Derek. And from that, the entire world changes. Would Meredith and Cristina have ever been friends if Meredith was a bright and shiny person?"
Rhimes hinted that some of the other characters' lives, such as Callie's, will also be markedly different in the alternate world.
"We had the best time figuring out what pieces of the world no longer exist because Meredith is a cheerful person," she said.
Private Practice star Kate Walsh, who is also returning to Grey's Anatomy for the episode, previously said that the concept is "really cool" and compared it to the movie Sliding Doors.