Alright here are some requirements when it comes to the Canon Servants that have already made a debut in the series. If you wish to be one of these characters, there will be some things on them.
1. I will need an RP sample, because I wanna see if you can play to the characters personality.
2. The noble phantasm of said character will be looked upon and I will decide how much Prana is needed, among other restrictions.
3. The stats and skills for the character aren't up to debate, they are set and final no matter what.
4. If you wish to become a Canon Servant, give me a PM, I will give you a template of the character for you to post, but you must give me an RP sample beforehand.
However, there is an exception. If you decide to use a Canon character who has the capability to be a different class, the stats and other things are up to debate. For example, if you made Heracles an Archer instead of a Berserker or Cu Chulainn into a Berserker instead of Lancer, I would consider them Custom and I would discuss things with you. However, Noble Phantasms still require debate.