Cause there's no way to actually keep all the threads in order as people answer them.

The Princesses:
1. How old are Celestia and Luna?
2. How old is Cadance? (Includes general backstory!)
3. Were Celestia and Luna always alicorns, or did they ascend?
4. Are Cadance and Twilight immortal?
5. Have there been other alicorns in the past? Will there be in the future?
6. How much authority do Celestia and Luna have in Equestria?
7. Does Shining Armor rule The Crystal Empire along side of Cadance?
8. Other than Twilight, Luna, and Cadance, what relationships have been important to Celestia in her lifetime (students, close friends, lovers, family)?
Ponies and Equestrian Culture:
9. Are there still cultural differences between earth ponies, pegasi, and unicorns, or is the culture homogeneous by the time shown in canon? Are there cultural stereotypes (positive or negative)?
10. Are there foods or items native to Cloudsdale that are uncommon on the ground? Are there foods or items on the ground that are uncommon in Cloudsdale? (Pegasus magic discussion too!)
11. Can all unicorns learn all spells, given the time and effort spent practicing them, or is magical talent usually limited in some way?
12. Earth pony magic: Does it exist? What is it?
13. Some pony families we’ve seen seem to have naming conventions (the Apples, Twilight’s family) and others don’t appear to (Rarity’s family, Pinkie’s family), which is more common? Are there reasons for one or the other?
14. How much formal schooling is an average, middle class pony expected to complete?
15. What’s the average lifespan for a pony? At what age is a pony expected to be independent of their parents?
16. Are there roughly an equal number of male and female ponies?
17. How informed are most ponies about things that happen in other parts of Equestria? What about other parts of the world?
The Main Characters:
18. How old are the Mane Six? Spike? The CMC?
19. Did Fluttershy remain on the ground after getting her cutie mark? Were she and Rainbow Dash friends all along, or did they drift apart for a while?
20. Rarity and Applejack both seem to have grown up in Ponyville. What were their interactions like before the show?
21. What do Twilight and Spike consider their relationship to be?
22. When did Pinkie move in with the Cakes? Is she a worker who rents a room, an apprentice, or is there some other relationship?
23. Who among the Mane Six had the best parents growing up? The worst parents?
24. Why didn’t the CMC hang out more/know each other before Call of the Cutie?
25. Is Scootaloo an orphan? Will she ever be able to fly?
26 (a). Will Apple Bloom’s cutie mark involve an apple in some way, even if it’s unrelated to farming or baking?
26 (b). How long before her parents's deaths was Apple Bloom born? How much younger is she than her siblings?
27. How is Sweetie Belle’s relationship with their parents different than Rarity’s was?
Side and Background Characters:
28. Is Mr. Cake the father of the Cake twins or not?
29. Are Big Mac and Cheerilee an item, romantically?
30. How did Prince Blueblood get his title?
31. Is Silver Spoon equally as bad as Diamond Tiara?
32. Is Zecora’s magic common among zebras?
33. Why does Daring Do publish her stories as fiction?
34. Do you have a strong headcanon for any non-speaking or one line background ponies that you want to share?
Other Species:
35. Does Equestria differentiate between speaking and non-speaking species, in terms of respect, rights and legal responsibilities? Is there some other line or scale used? (For example, how are cows classified? Diamond Dogs? A chimera?)
36. Do other speaking species form their own nations, or are all nations largely integrated, with some having larger populations of a species than others?
37. Are minotaurs related to cows at all?
38. Do all zebras rhyme?
39. Do zebra cutie marks work the same way as pony cutie marks?
40. Do any species have types of magic, other than ponies?
41. Is the lack of buffalo presence in Equestria cultural, or simply because most pony towns aren’t built to accommodate a full grown buffalo?
Headcanon and You:
42. Do you consider official sources other than the show (comics, card game text, movies, novels, etc.) to be canon?
43. Do you have one headcanon that you always use, or do you switch depending on projects?
44. Can you easily accept the headcanons of different writers? Do you prefer when stories match your headcanon?
45. Is there a part of your headcanon that you wish was more popular among writers?
46. Is there a popular piece of headcanon that other writers use that you dislike?
47. Have you ever written a story or blogpost just to explain some part of your headcanon (other than this one)?
48. Does your headcanon influence things like your favorite pony, ship, or episode?
49. Would it bother you if the show proved part of your headcanon wrong? Any specific piece that would really bother you to lose?
50. Is there something that wasn't asked about here, but you feel like you need to explain to everyone?
New Questions
[Ponies and Equestrian Culture] Do other kinds of pony exist beyond alicorns and the three tribes? And how do non-pony equines, like zebras, fit in?

Please let me know if any links are incorrect!