This can be applicable to both PvP and PvE.
Instead of a leaver going black and disappearing, when someone leaves, let the default henchmen AI of the primary profession take over.
This means that the party with a leaver does not suffer one less character. When a player leaves, his Heroes get left behind, so why not the character himself.
This solves the problem of the other party leaving in Vizunah and Unwaking.
If the player had a convoluted bar, thats too bad, another human-shield is better than nothing. Any exploit to this is the same as having a Hero. So, you can make super Interrupters and stuff like that.
Just put a tag after the name like "Blah De Blah [AI Controlled]" or something.
All factions, exp, drops and gold shares are wiped.
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And the remaining people can hurl abuse at the leaver's puppet. You can also take this opportunity to /report the guy.