Normally, I'd just make my own OC's for this story (mostly background characters, tbh), but I like the challenge of writing my friends.
So!
If your OC is:
-A Royal Guard (either Luna's or Celestia's)
-A Medic
-A Weather Pony
-A Good Samaritan (from cities BESIDES Canterlot, variety encouraged, the type to happily carry supplies to a different city)
-A Pony Maybe You Don't Mind Being In Hell, If That's Okay With You