I've thought about that one before, but:
- MyBB's editor is "broken" (like many parts of MyBB itself) - its OOP model is crap and you cannot have more than one instance of it on a page; in other words, this will screw up the normal post editor if I put it there
- a WYSIWYG editor can be quite a large additional library - add to the fact that it only really does HTML well; MyBB itself doesn't have one, which is going to make it more difficult for me to include one, especially if using MyCodes. The best I could do with this idea is perhaps integrate it with a WYSIWYG plugin, but all the ones I've seen so far suck in some way or another at the moment.