When the topic change is sharp enough, the reply-as-new-thread should sometimes go to another forum altogether.
Maybe put in that forum-selection combo-box (like on the front page in "New Forums Only" view) next to the "As New Thread" check-box, to decide which forum the new thread goes to?
It would probably have to default to the current forum, and I think this one -- as opposed to the check-box itself, per the above -- really ought to retain its state when you go to Preview.