Vitor: What about extra layer to adopt mainstream? Today it's pubkey/private key. What about "brainwallet" way? Example: email and password. It would guarantee unique "brain wallet" key. User would not need to know/see that it creates a private/Public key.
Discussion
Impossible without centralizing the conversion between user + password into the private key.
Just to give you an idea, without centralizing it, the "e-mail" would have to use 22 words to represent it. It would be VERY long.
No. Use same logaritm as brain wallets use. Example: https://brainwalletx.github.io/
There is no lower or upper limit to create a private/public key. You can create a key pair using a single space even.
Also, you could support this in Amethyst alone and let the user see/get his private key after login, to be used in other clients.