-Don't overpower (godmod, ad weird powers that don't canonically exist, weird hybrids that don't canonically exist, etc.)
-Do not make them related to any canon characters
recommend not putting them in any relations with the main six characters or any other important character (e.g. the princesses, shining armor, cmc trio, etc.), buuuut if it's that
important and/or necessary.. alright. But good luck.
-No super weird cutie marks. Don't use overused stuff, nothing already used on an important character in the show, and make them unique and make sense!
-No ponies w/ effed up personalities (e.g. emo, crazy, etc.). That's weird. Stop it.
-No possession, no grim dark junk (but that's imo... still no possession tho. dat is a nonono!!).
-PLEASE for the love of GOD do not put inappropriate gross nsfw junk with your OC's! DAS NASTYYYY
But imo, w/ alicorns there are some exceptions (don't judge me aaaa)...
I have two alicorn OC's that I never use... but they're just there because I think making up old lore is cool. One is the Alicorn of Protection, and the other is the Alicorn of Hope. They're just normal alicorns but aren't rulers of anything and are basically "myths" (Basically, if a pony needs protection or hope, the pony can "call" on those alicorns as if they were deities, if the pony chooses to believe in them.). So really.. they aren't real.. in a sense. It's stupid and complicated, I know. It's kinda like Okami or something, where if you believe in them, you can see them. And if you don't, you don't see them.
So yeah... don't hate me.