Changing textareas to editable div (alternatively, implementing toggling between textarea and editable div) would (probably) easify working with emoji and also would be a step closer to wysiwyg editor implementation.
It would have almost all of the inconvenience of using a rich text editor with none of the benefits. Anybody who thinks it would make anything better needs to describe specifically how. Just replacing the textarea with an editable div does nothing by itself while breaking anything that relies on the textarea API.
The outcome of such elements would always be HTML - it's like one step after having completed a WYSIWYG editor for phpBB. You should also provide an idea/implementation on how to perfectly convert the submitted HTML into BBCode again.