Hi, the features you are asking should mainly be done in the email application itself (As opposed to the crypto backend). So if you are using Enigmail, you should send them your suggestions.
E.g. KMail/Kontact can save certificates for each contact, so the certificate selection is simplified.
Note that for the featue of passphrases there are several solutions on the side of the crypto-engine: The gpg-agent is caching the passphrase if you want to - you could extend the time period until the cache is cleared. You could also create a key without a passphrase (for some automated causes) or use a smartcart (which is supported by Gnupg):