It would be nice if we could combine the various "feed.php" modes with a specific subforum, eg:
http://forum.freegamedev.net/feed.php?f=53&mode=topics - Show only "new topics" in the subforum "Red Eclipse"
Currently, you can only specify either a subforum, or a feed mode (so you can only get latest posts within a forum, or only new topics for the entire FGD forum).