Creating a discussion would move you to a page where the discussion will be as if when it's posted. This will be a client-side only page, and would work the same as the preview post feature. I really miss this feature too aswell, and I think that it should be a core one. Why would we be able to preview posts, but not new discussions? My suggestion would require some modifications in extensions too, I think (for example, tags extension adds the tags to the discussion, which would have to be also rendered).
Another idea would be to just add WYSIWYG editor to Flarum. Some extensions already tried this approach, and this one seems quite up-to-date, however it doesn't seem to get much attention, which could make the developer not passionate about it in the future, but if it'll be worked on, it's gonna be fantastic and I really like the idea: https://discuss.flarum.org/d/25590