Great post ! I've got a few NPC's that have been kicking around and developing for a very long time now.
One in particular came to the fore a couple of years ago, she was very much the fixer type in the town she is based in. The party passed through, spent a lot more time there chasing red herrings than expected. I thought it would be a shopping and re-supply stop that would take about 15 minutes of play; they ended up being there for nearly three months.
Now, one of the characters gets a tick every time the name "Merielle" comes up, growls and says she'll go back and settle scores one day, and the player isn't much better. I'd say that's job done.....
Your players holding a long-lasting grudge is like a gold medal of character creation and world-building, congratulations ;)