"What if a bunch of authors crashed on an island and discovered (a) character(s) from stories they had written?" I haven't seen the idea before, but if it's been done then that's unfortunate.
EDIT: I've decided to stop comparing it to Lost, as that seems to confuse people.

The Roster (as it stands, subject to change):
Solar_Flicker (Kiana/Light Steps/Willow/Inkwell)
Tobyc (Ember)
xnecrospherex (Galeful Night)
Corvo (Jason/Starry Skies)
hccpu (Twisted Logic/Oren)
Merc the Jerk (Jack Apple) Humanized
JJ Gingerhooves (Pinkie Pie) Humanized
Barrobroadcaster (Michael Hay)
The Rules
1. Each author may have 2 main characters, one of which must be from the FiM universe (pony, gryphon, diamond dog, changeling), both preferably. You can also have only one if you prefer, but it must be a pony.
1.1 Rule changed. Can substitute self insert with another character under special circumstances.
2. No shipping may occur between the author and the story characters.
2.1 Rule changed. No shipping that would otherwise break the idea of the story. (No interspecies shipping basically.)
3. It should be obvious, but you can't use the main characters from self inserts.
4. No OC alicorns. Celestia, Luna and Cadence are fine. (Twilicorn is fine too... I guess)
5. Every character will have their powers "stripped" from them (anything they would not normally have, unicorn magic/pegasus flight are fine). They will retain their knowledge and any items it would be realistic for them to have on their person.
6. Humanized ponies count as ponies, I suppose.
7. How the characters (not you) get to the island is up to you but it can't be directly caused by another character. Sleeping or awake, on a raft or in a tree, buried in the sand or in a cave.
8. IMPORTANT: You have to make an appearance at least once a week or explain your absence. It's a way to keep track of who is still in this or not.
(There can be exceptions, but they have to be cleared by the rest of the collaborators.)

We even have a google document!
https://docs.google.com/document/d/1HWjr2Qa7aY-wBay5267mED4APkGDibc7hWPLDHivCb8/edit?pli=1

Also, here's some information about the island inhabitants:
http://www.fimfiction.net/group/198212/one-lost-island-the-collaboration/thread/17203/nice-information-about-island-inhabitants