I came across this thread and I just wanted to point out that this appears to already be a thing provided on the forum. Though it is not as clear as you may notice in ProBoards. To toggle between the Code View (default view) and the Modifyable Preview (toggled) modes, you just need to press the 'Toggle view' button.
(http://i.imgur.com/8rDgACh.png)
When you are in this Modifyable Preview, and you want to remove a format you added, then you don't have to toggle between modes in order to do so. You can just select 'Remove Formatting' after selecting an area that has formatting applied. You find this button directly to the left of the toggle view. I'm not sure if this is what you were talking about, but what you're describing already is provided on the forum.
(http://i.imgur.com/HggZUtC.png)
Here's a quick preview of the second view in action:
(http://imgur.com/GG1eSFG.gif)
Note that the above preview was just tested with size and colour. Anything provided in the formatting area will work in the toggled view.
However, if you're wanting to add an unlisted colour, font, or spoiler, you may have to toggle back to the default view in order to make those additions.