Introduce a Thread View Mode while reading messages. Use '*' to toggle.

Any alpha-char command or entering a message number will exit thread view
mode. If the entire thread fits on the screen, then the arrow keys can be used
to move about the thread (up, down, left, and right). The old thread-ID keys
( and ) still work, but a little differently. And while in Thread View  Mode,
ENTER  and - keys now move forward and backward by thread.

I meant to do this a long time ago and really should have (all the underlying
thread support was there). It really highlights all the network nodes that
don't support REPLY-IDs. :-(