It's fun to think about indeed, but in terms of gameplay I suspect it would actually make the gaming experience miserable. IRL, people are already working all days most of their life to reimburse their various debts, transposing that mechanic in game would be terrible.

People are already complaining about “having to work” to cope with the gold sinks in MMOs, I don't think many would find it fun to be tracked by repo men in game.

probably true.. I'm not sure how you'd gamify that... where both parties would enjoy that.

This isn't too dissimilar from the Covenants in Dark Souls, which influence the way players match with each other. When you kill an NPC or commit other heinous acts, you gain Sin, which any players of a given Covenant can "invade" your world instance at random anytime they want to punish sinners.

This covenant system created a deep and engaging playerbase. I've not seen any other game pull such a system off, but it was fun. If the risks and rewards are right, it can work.