Would it be a good idea to be able to 'tag' old threads when starting a new thread?
So when you start "Penguin Bollards thread 23' you can tag any of the individual threads in a series of threads and and all the contributors receive alerts etc. as if it was the original thread they'd contributed to.
Tag the first in a series of threads and all the people who've contributed to 'daughter threads' would be included in new alerts (unless they've unwatched etc.)
Maybe even an automatic link in a '1000 posts, thread closed' message to start a new automatically-named thread and add a link to the former thread as the 1001st post?