Mental Wounds.
Attacking another character wounds the attacker with "Regret 5" which can be healed via gauze-like only foragable. Or gauze itself, doesn't really matter. The point is: one at a time.
Killing another character inflicts 10 x "Mental Anguish 5" with a -1 debuff on int, str and agi each. Can be healed one at a time with something a bit more rare. No idea what yet but let's say an "ulta-gauze" that requires 2 wool and a silk thread(or cloth).
Pvp lovers will still be able to pvp others with a little penalty. Whole system brings a new thing into the game: Robbery. Since you can attack others, KO them and rob but killing is a dire option.
Alts.
Since you guys brought XP system in, I assume, you're against alts. If that so, there is a really easy way to get rid of them nearly completely. I'll pm you how if you're interested.
Social aspect.
Both implementing mental wounds for killing and removing alts gives the game a stronger social part.(I'm saying stronger, in fact it gives the game a new part: social. There isn't one right now)
Since we can't excel at everything anymore we need to cooperate with others. And since KoS'ing isn't a thing anymore due to high penalties, that becomes a lot more easier and fun to do instead of being constantly afraid for your life. Just a quick example: a guy proposes a trade. Right now, instead of thinking about the trade, I'm thinking "that bastard wants to lure me out and kill me". In the new system I'll think "damn, mb he'll rob me..?" and, well, that's a lot more fun than being killed tbh.
I just woke up so I'm a little rusty, I'll update the post once I get myself together.
Suggestions are welcome! Thanks for reading.