Hello!
Thanks for your work on this, this extension is really a very needed thing for many users including me.
As for features, I have an idea about some feature that I know will be appreciated by many Russian and probably not only Russian users, even though it's probably hard to realize. I'm talking about "Mail dispatcher" tool.
This tool was introduced by Russian company RitLabs in their well-known in Russia mail client "The BAT!" and later something like this was present in "The Bee" program.
To be short, this tool allowed user to control his local mailbox without loading all incoming e-mails into it and worked like a manual spam filter (however it was rather powerful tool). You could see all messages titles list in a remote mailbox and select which messages you want to load locally, which you want to remove without reading, which you want to leave on a remote server yet, and which to download now (something like Thunderbird's and Simple Mail "load only titles/headers" feature, but much more convenient).
Once there was a long discussion on a Russian Mozilla Community forum with a poll, and 73% of all, who voted, would like to have this tool in their e-mail client. A Bug 488001 was created, but no one of the developers agreed to us, so the bug has got RESOLVED WONTFIX status.
If Simple Mail would have this feature, it could make it very attractive alternative for many users, who currently use "The BAT!". However, this would be a heavy thing, so I believe, it could be realized as a separate extension for Simple Mail (like separate extensions for Firebug, for example).
It would be really great if such extension existed!

1. IMAP IDLE
2. button to auto-report spam to spamcop (by forwarding as attachment to predefined email address) - a-la Mailwasher
3. option to suppress the confirmation when deleting messages - DONE

1, http://en.wikipedia.org/wiki/IMAP_IDLE
Basically, push of the new messages by the server to the client, so the client receives them immediately rather than on the next send/receive.

2. mailwasher, where this is implemented, is very light client; pre-client, if you will. (at least its early versions were light)