Rarity Takes Manehattan · 9:24pm Jan 6th, 2014
So the latest episode ends with Rarity receiving a spool of fabulous thread, shimmering rainbow-colored thread.
It could be coincidence. Or it could be a call back to the end of Season 3, when Rarity possessed Rainbow Dash's cutie mark.
Finally, there could be another explanation for this thread. I was inspired to create a short one-shot: Rainbow Thread.
Except instead of any of the above, it's her Rainbow Key, for the box that was found in the season premiere. Really, stay current.
1690446 Damnit, I was going to say that!