This is something I support. How this idea best work, I'm thinking of something like...
1. Thread becomes "time-locked" after about half a year of inactivity.
2. Icon for time-locked threads will be shown with a green "zzz", rather than your typical golden lock. It will also function a different way from your typical golden lock from mods.
3. OP can still edit their posts in the thread (useful for things like art threads, tutorials, etc). This doesn't mean they can edit other people's post who had quote them before and etc.
4. Thread becomes unlocked (for everyone else to post in) once the OP posts in their time-locked thread.
5. Once time-locked thread is unlocked by the OP, there will be a small icon/timestamp indicating its revival in the first post (automatic feature from Gaia, rather than OP manually making note of it).
6. Thread search feature will have a pop-up indicating how old the thread is, but you will still be able to see the time-lock key. If users ignore that, they will still find out that they can't post in said thread.
7. Non-OP of time-locked threads can PM the OP for thread's revival, or discuss about said thread's topic via PM/comments/etc. For friends, roleplay, etc, I'm sure that the person you hung around with wouldn't mind. Heck, you guys can still communicate via PM, or decide to just hang out in another thread/guild/chat-area/etc.
8. If user doesn't want to PM OP to post in inactive thread, he/she can simply recreate the thread.