That's a good point! I hadn't thought of that angle.
Flarum's preview feature strikes me as an elegant solution that provides for both users who want WYSIWYG and those who want to see the actual Markup and/or BBcode. Once previews are available for new posts as well as replies (which Toby has said will happen eventually) then everyone should be covered.
chirag Can you be more specific about what sort of WSIWYG function you want to add? Also, have you tried using the preview function when typing in replies?
EDIT: If you're hoping to add the Preview function to the new discussion composer (as I guess may be the case, now that I've read the title of this thread again), then you should probably check with @Toby and see if he has done any work or had any thoughts in that direction. I know he was planning on adding this feature at some point, so he may have some pointers for you. (Assuming you'd be interested in contributing code to the project, of course.)