It would be great to create your standard email signature by picking the font, size, color, etc. same as Outlook without having to use HTML. I would also like to add images without HTML as well.
No one says you have to code in HTML and including an image is as simple as it is to add it to an email. HTML is the file type of internet mail, the choice to use Thunderbird composer to do your HTML coding and just select the correct file type when you save or to craft it by hand it your choice.
One thing I sincerely hope no one ever considers is using Microsoft Word as the editor like Outlook does. My emails are large enough, without a copy of Words normal.dot files crammed in there and a load of other proprietary Microsoft binary data.