I’ve always liked the idea of showing the post preview inline where it’s actually going to render… I think that could apply equally well on mobile and desktop (even if the side by side preview remains on desktop).
That way, when you’re on the topic page, you’d see how your post is going to look as you’re typing it. On mobile, minimizing the composer would give you a live preview too.
However I agree with Sam when he says a Full screen composer is the way to go. Then I must say that this is quite a “technical” kind of forum but many other “different” and more casual forums can have great benefit for a streamlined insert of images in the posts for the users that really needs an uncomplicated and clear way to partecipate.
Many user are not as tech savvy as the admin of a forum or a developer, or a participant of a tech-related forum etc.
And as today We write more with mobile phones and We “talk” much more through images, this is a fact supported by every statistic You will look at.
I really love Discourse and I think this will be by far one of the most important aspects to make it more appealing to a big mass of public.