When a player logs out, their character in-game "sleeps", and is not removed from the persistent game world.
The sleeping character can be interacted with - a "capture" function will be available. Sleeping character cannot be killed (or can they? thoughts on this?).
This mechanic would give players a very strong reason to have a safe place to sleep, making strong defenses meaningful. Solo players must live in more hidden/out of the way areas in order to survive.
Capturing a character will allow the captor to lead this character wherever the captor wishes (there may be additional functionality with a "handcuff" type item that could limit the captive's abilities). Captives could be kept in prison cells until they awake. Captives could be rescued by their guild, or released by negotiating a payment with the captors . They would not spend time waiting in a cell unless they wanted to - freedom would always be available through ransom or suicide (perhaps the skill-loss penalty could be raised, so it does not become the "go-to" method of escape).
So why introduce this new mechanic at all?
I'll try to be very brief:
1. NO OTHER GAME OFFERS THIS. Life is Feudal is such a unique game, it is one of the few if not the only game that could support such a mechanic (and I argue benefit from it!). This would further establish LiF:MMO as a unique game and draw more players to it. "Dangerous" games are becoming more popular - PUBG, DarkSouls, etc. People want a rush!
2. REASON TO BUILD. It would give more meaning to building a strong settlement and maintaining it. Sometimes it feels like big strong castles and fortifications are built for looks. That's fine, but let's add some functionality and value to them!
3. MORE CONTENT. This mechanic would also bring guilds together more to rescue captured friends, raid other settlements and capture their inhabitants.
To those who don't think it would be fun to be locked in a cell: Are you really having more fun hitting a dummy the entire time you're in-game? Being captured would be a rare and exciting event. Better than being bitten by a snake.
Unanswered questions: What about players who dc? Do I have to get back to my house every time I log off?