Have the system automatically populate the title, but allow it to be overridden. Make overriding this default painful (e.g. after submission you have to open the thread, click edit, enter the new title, and give a reason for the change - then 5 community members with over 1K kudos need to vote yes to approve this change). Most people will fall into line in not changing the titles, but you have the flexibility and take the pain in the rare cases when it's needed.