Like The One Where Everybody Acts Out Of Character (which despite the order I'm putting these up, you should not run right next to this) A Whole New World takes advantage of the flexibility and kitchen-sinkiness of the Buffyverse so you can take a break from the regular game while still playing it.
It lets you spend a week in an evil mirror universe where half the Cast are vampires, a future cyberpunk dystopia, a visit to
The Cast might be mentally fitted in, making this One Where Everybody Acts Out Of Character as well. That can be fun to play, particularly when it doesn't affect everyone and the one or two sane PCs have to make the others see sense. And if they're up for it (generally best to check in advance) some of them can play the alternate PCs all the way through, even acting against the heroes trying to set things right.
They might also have to dress the part, particularly if it's embarrassing and/or likely to turn heads. Oh, and if you cast your characters, now is a chance to use a photo of a given actor which totally does not suit the character they're cast as. Indeed, sometimes I've been tempted to do A Whole New World episode because of such a picture.
And if you want to add a maudlin element, one or more of the PCs might find the Whole New World appealing, especially if the real world is treating them like crap at the moment. Harder to do with the post-apocalypse-Hell world than the everybody-gets-what-they-always-wanted world, obvs, but even there what if you fall in love with someone who might never be born if you put the world to rights?
Example: Friendly Neighborhood Vampire Slayer
Heading out on patrol, Billie finds herself being followed by reporters. Who all want to ask her about slaying. Then a kid asks for her autograph. Then the crew from that paranormal investigation show come over to get a piece to camera. Then she gets a call from the Mayor's office about getting the key to the city tomorrow.
Some of the band seem unfazed by suddenly being as famous as the Ghostbusters, like it's been this way for ages, while some are aware that they never used to have a police liaison and the news stories of their heroic exploits which go back months didn't exist last night...
(This could also happen gradually in a world-changing development, as partially happens in Season Eight.)
(Alternate version which is way too close to The One Where Everybody Acts Out Of Character example: The band suddenly find they're at number one, scheduled to appear on the MTV awards, being covered in a special episode of glee, mobbed by fans everywhere they go...)