This is the thread in which we post questions designed to help you get your creativity muscle flexing. These are updated frequently.

Hidden beneath the old tree library, was an ancient vault. What would be hidden there?
A certain somepony (not Rarity or Sweetie Belle) has a secret crush on Spike. Who could it be?
How did Starswirl the bearded disappear? Did he die? If so, how? If not, where'd he go?
Why did Celestia abandon the ancient castle and head to Canterlot?
What would happen if Twilight learned a spell that could pause time for sixty seconds from her perspective?
Twilight needs assistance with something, but the nature of that assistance is a little awkward for Spike. What is it?
What would happen if one (or all) of Applejack's trees started growing oranges instead?
Where are the rest of the diamond dogs? What does a female diamond dog look like? What's their diet?
Why was Canterlot built on the side of a mountain? The view? The resources? Trying to escape the smooze?
What are the elements of harmony? Gems? Personality traits? Magical artifacts? Magical essences? Reincarnating spirits?

1) Genealogical records of Starswirl the Bearded's descendants up to 100 years ago.

2) Apple Bloom.

3) I have two ideas for this one:
A) Starswirl decided to settle down and live a quieter life, eventually starting a family.
B) He got stuck in the Equestria of Equestria girls, during the Medieval Ages, where he helped a little boy become one of the greatest, if not the greatest king in history.

4) The castle was too damaged, the surrounding forest was corrupted by the dark energies of the Nightmare, and, of course, too many painful memories of Luna.

5) A potential combination of Lesson Zero and It's About Time.

6) Dragonic sexual development. Aside from the obivious, this is awkward because Twilight would have to give Spike "The Talk".

7) If it was one tree, Applejack would be dumbfounded, and would most likely blame Discord and have Fluttershy try to get him to change things back. If it was the whole orchard, she's likely faint and, when she recovers, hunt down and beat the ever-living daylights out of Discord, who she'll blame for this.

8) Fido's group was a rouge group, most Diamond Dogs are about as nice as Equestrians, if somewhat more xenophobic. Most Diamond Dogs live in a secret network of massive caves (think The Underdark of D&D fame). The females are a little less stocky than the males, but no less strong. Diamond Dogs are primarily carnivores, subsisting on subterranean lizards, bats, and fish, but also supplement their diet with roots, fungi, and subterranean plant life.

9) Canterlot was originally a simple fort with a small surrounding village on a ledge on the side of the mountain, but it was quickly expanded after Celestia declared it the new capital (it was originally supposed to be a temporary thing, with Trottingham being the intended new capital). Canterlot was made the way it was in order to make it something truly unique to any city in Equestria, and the world.

10) The Elements of Harmony are actually the fragments of The Rainbow of Light, given strength by the friendship between a Unicorn, an Earth Pony, a Pegasus, an Alicorn, a baby Dragon, a Sea Pony, and a Flutter Pony. Some versions of this story stat that there was a Seven Element that was wielded by a mysterious being known as The Magnus.

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1. Hidden beneath the old tree library, was an ancient vault. What would be hidden there?
Various letters and artifacts from another period of time before ponies existed
2. A certain somepony (not Rarity or Sweetie Belle) has a secret crush on Spike. Who could it be?
Diamond Tiara
3. How did Starswirl the bearded disappear? Did he die? If so, how? If not, where'd he go?
He eventually died of old age after retiring from Magic. His last days were spent living with Celestia in her new home post Nightmare Moon
4. Why did Celestia abandon the ancient castle and head to Canterlot?
The old castle held too many painful memories of her and her sister- she didn't feel it necessary to try and rebuild it to former glory and instead headed elsewhere.
5. What would happen if Twilight learned a spell that could pause time for sixty seconds from her perspective?
She could evaluate a life changing decision that needs to be made on the spot, and try to think the options through a bit.
6. Twilight needs assistance with something, but the nature of that assistance is a little awkward for Spike. What is it?
Anything involving where foals come from. Or trying to figure out any history involving dragon lore
7. What would happen if one (or all) of Applejack's trees started growing oranges instead?
The family changes its main source of income and sells oranges. Then they invite the Oranges from "The Cuitie Mark Chronicles" to invest in the farm.
8. Where are the rest of the diamond dogs? What does a female diamond dog look like? What's their diet?
They all live in that cave. Don't care. And they eat gems and dirt.
9. Why was Canterlot built on the side of a mountain? The view? The resources? Trying to escape the smooze?
The mountain has existed since the founding of Equestria. The denizens see it as a symbol of power and hierarchy, and so they live on its edge, basking in its glory. Also, the view is awesome.
10. What are the elements of harmony? Gems? Personality traits? Magical artifacts? Magical essences? Reincarnating spirits?
They are gems that encase the spirits of six young girls who were best friends during life. At the time of their death, they became the elements, and link the bearers together in friendship.