Message templates/stock replies
I keep a collection of form messages in a Gnote note, such as (paraphrasing) “Please remove the following postal address from your distribution list” and “Dear recruiter, no thank you”. Currently I dig this note out and copy-paste the relevant message into Geary. It would be nice if this could be integrated into Geary itself.
How would you like it to work
Good question! One requirement would be that the template/canned message is available when replying to existing threads, not just when starting a new thread. It's been a while but I believe Thunderbird supports templates; however, they can only be used when composing a "fresh" email.
Relevant links, screenshots, etc.
Looks like I was right about Thunderbird – this article has links to various extensions which behave roughly as I was thinking.