I’m just another end user in this matter 
But until we get WYSIWYG at some poin in the coming future, another toggle button wouldn’t be any issue in UI/UX wise — coding such functionality would be totally another story. Even WordPress has that show code button (editor system is awful mess in WP, though. It broke totally when Automattic decided go to look first, content last vision).
I still see that the most important thing would be a setting to hide toolbar totally, except file uploading. Mobiles have it already (as an eastern egg 
), though. I see that more important to do than editor improvements or changes.
We have two different user categories:
- a huge majority that almost never write anything and need really low threshold (is that right word…) to comment
- power users who need a big variety different tools and easy access to those
But of course that depends of niche of forums. But my bold claim is that is true on majority of forums.