I've been exploring possibilities relating to your request. AFAIK, there is no way to do exactly what you request since the "New Threads" list has no limiting factors as determined by the user.
However...
When you view any particular forum, you are automatically viewing the most recent posts to that forum. As long as you aren't subscribed to too many, it shouldn't be much of a problem to just view them this way.
HTH!
-Dave