11.18. It would be great if Enigmail could do this-and-this! Could you please implement it?

You can submit feature requests in the Enigmail Forum, Feature Requests thread. To know how to reach the Forum, see Section 13.

But please first consider that Enigmail follows the OpenPGP standard. It is not its purpose to innovate or invent new protocols. If the feature you propose is not included in or not compliant to the standard, the feature is not going to be included in Enigmail, no matter how many users ask for it.
The Enigmail source code is freely available, though. If you really need such a feature, you can download the code and modify it to suit your needs. Please consider first that breaking standards is generally not a wise idea, and will result in incompatible products.