Okay, listen to this:

All of the Mane 6 get stuck in Twilight's brain, then Pinkie's, and so on! Thoughts?

2544677
Twilight's brain would the the most logical and organised, probably nothing really dangerous there, except of the possibility of sensory overload when she starts to think hard about something.
I'd write Pinkie part as the final one. It'd be weird...
Fluttershy's brain - nothing scary at first, a meadow, trees, flowers... Then the wild animals appear. Or maybe Flutterbat. Or "you're going to LOVE ME!" Flutterrage Probably everything at once.
Applejack - seems ok, but why are we force-fed apples?
Rainbow Dash - flying, more flying, flying till you get motion sickness, Soarin. Parts of Soarin rest of the mane 6 wouldn't want to see.
Rarity - "why are you in my brain?" then a standard romance plot she daydreams, till the rest of the mane 6 starts to feel creeped out.

2544677 How can Twilight get stuck in her own head? I'm having a hard time picturing that. It's like an endless recursion going on.

2544965

Yeah, she wouldn't exactly be stuck in her head, but the others would.

2544876

This would be an "everyone" rated fic, by the way.

2544965
Hmm, technically everyone is stuck in their own head, I think (it's almost 1 AM here, now I'm gonna think about it for the rest of the night...)

2545016
Ok, so no Soarin. And romance plot being more romance and less plot

2545049 We seem to be in the same time-zone. Have a pleasant recursive dream fellow time-zoner.