The following is a list of essential requirements that an editor would have to meet. If we think of anything else we'll expand the list.
- The WYSIWYG editor has to generate valid BBCode that phpBB can handle
- The WYSIWYG editor should not show anything that cannot be represented in BBCode (e.g. aligning right if there's no bbcode to do it).
- Custom BBCodes which a phpBB administrator defines should be available in the WYSIWYG editor and be displayed.
- Smilies should be displayed in the WYSIWYG mode
- All tags should be shown in the way they will look in the final result.
- It should be possible to position attachments in the post
- The design should be easily adjustable to the used phpBB style.
- The entire editor should not have too much code so that it loads almost instantly and users don't have to wait for the editor to start writing their post
- Maybe URLs that would automatically be detected and turned into links should be turned into links in the WYSIWYG editor as well.
Which of these features are going to be internal parts of the editor and which ones will have to be added as plugins is up to you. It just has to be necessary to do so without any necessity for changes to the editor code itself.