What about text snippets? This would allow some ideas above to be implemented by users (like a keyboard shortcut to strikethrough). There is this topic, but I am not sure what happened with the idea.
I really would like arbitrary code to be entered, because it would allow defining custom html code in the userstyle.css. I have no idea of how hard it would be (maybe it is really hard and that is why nobody did it!).
Maybe I am spending too much time using emacs and yasnippet.