The "Coming soon to a computer near you" option looks good!
I would suggest some arbritrary automatic split of threads (again, 50 sounds good) but I don't know how much programming this would require.
I can't agree with Louis Roy, some threads rapidly reach 100 postings without exhausting, and some before the other side of the Atlantic is awake.
Other threads live on, and are continued 2 or 3 years later by those who make a point of searching for the information they require before starting a new thread. They then post to the old thread so that everyone can see what has gone before. So, to quote Marie Antoinette, a guillotine is not necessarily a good idea!