I would like to be able to type short text sequence in the editor, and have those characters replaced with a predefined (larger) text string, not in the rendered text, but in the editor itself.
Does this functionality already exist in Joplin or a plugin?
Doesn't answer your question, but I do this kind of thing using the Text Function in a little program called Keybreeze. I type a hotkey followed by something like "ys" and then the space bar, and out comes "yours sincerely". It works in the Joplin editor.
Yes, I believe this functionality is out of the scope of Joplin. If someone creates a plugin, by all means.
But I agree with skim1124. There are many such tools available (on all OS). If those tools didn't exist, a plugin would be the way to go, but since they do, it's probably a waste of time.