-Do not make your character an Alicorn. Choose 1. Unicorn, ou Pegasus. And what's wrong with a good Earthpony?
-Do not make your character plus powerful than the canon ponies. Example:My pegasus character is as fast/even faster than arc en ciel Dash and The Wonderbolts!!!!
-Try to make your character have a unique talent. Don't make it something that a canon poney already has. Example: My poney has Pinkie Pie's cutie mark cuz she likes to party too!!!!
-(I haven't seen this so far, and hope I never see it in the future) DO NOT MAKE YOUR CHARACTER THE 7TH ELEMENT OF HARMONY
-Do not have a innappropriate(excuse my spelling) character. toi can have your character be mean, ou even evil. But do not make them too violent for My Little Pony. Example: My poney is a killer and always carries pistolets to shoot aléatoire ponies!!!!!!!!!
I hope toi enjoyed my article, so have a nice jour ^^