It is possible to create custom sign up and sign in forms (email, password). However what is not possible is to create a custom sign up form, if you want to use Single Sign on with Google or Facebook. Since Single Sign on is really important nowadays, I think this is an important feature to release. What we need are Google and Facebook buttons that start the SSO process and some corvid function that returns the information retrieved from the SSO process.
I know that there is SSO in the prebuild Sign up form, however I guess a lot users of Editor X want to have there own design and flows throughout their site and the Sign up / Login is quite important.