I just watched Rarity Takes Manehattan a few days ago, and I wanted to share some thoughts.
Like a lot of people or ponies, I think that rainbow thread is Rarity's key. It also hints that the next pony to get her key is Rainbow. It seems like every other episode, a key is found.
Rarity Takes Manehattan--Rarity got her key
Pinkie Apple Pie--the 'inbetween episode'
Rainbow Falls--Rainbow has to choose between Ponyville and Cloudsdale, which will show loyalty (most likely to Ponyville). She will probably get something pink as a reward, hinting that Pinkie is next.
Three's A Crowd--another 'inbetween episode'
Pinkie Pride--Pinkie needs to be the best partier, so she has to get everypony to laugh, showing the element of laughter. She will get her key.
So, without even reading anything, and just by watching Pinkie Pride, we should be able to guess who is next. Although this is just a hunch, I read twenty one comments to get these ideas. So, we will know when the episodes come out! I am really excited.
Rainbow just got her key, Spitfire wonderbolt pin. So, most likely the key will have a rainbow flash. This disproves my previous theory, we won't be able to tell who is next, Rarity's key just so happened to do so. But no matter what, my other theory on every other episode has not been disproved yet.
Gosh, this update is LATE! Well, anyways, Pinkie Pie's key should be the chicken, Boneless given to her by Cheese as a gift for when she gave him his cutie mark by being the best partier or should i say ELEMENT OF LAUGHTER. Anyways, three down, three to go! Hurray!
Well, in Filli Vanili, Fluttershy should've gotten her key. I don't really see how it showed kindness, so I'm guessing since half the ponies got their key, they're going to play two inbetween episodes. Although it's also possible her Pony Tones suit is her key, I don't see how she got it by being nice.
Happy March, everyone! I guess Fill Vanilli WAS an inbetween episode. Anyways, is that flower looking suspicious or what? Fluttershy got her key! So, now we have...
Rarity (spool of thread)
Rainbow Dash (Wonderbolts pin)
Pinkie Pie (Boneless)
Next will be Applejack and then Twilight, I'm guessing.
For Whom The Sweetie Belle Tolls? Really? Anyways, neither Applejack nor Twilight are making an appearance anytime soon. :(
We'll have to sit back and watch.
Flim Flam is back! And as usual, AJ does not like what they are bringing. :P. Anyways, either way AJ finally got her key! And of course, it just happens to be a coin. Well, we should be expecting Twilight's big magic or big friendship act that gets the sixth key! Here is the (almost) final list!
Rarity--"Rarity takes Manehattan"--Spool of Rainbow Thread
Rainbow Dash--"Rainbow Falls"--Wonderbolts Pin
Pinkie Pie--"Pinkie Pride"--Boneless the Chicken
Fluttershy--"It Ain't Easy being Breezies"--Flower
Applejack--"Leap of Faith"--Bit