I like the "unscripted villain" myself.
Just keep notes on any surviving NPCs from adventures you've run the PCs through and see which ones your characters reacted strongly to—usually I keep a notepad file on NPCs. If one seems to spark interest I'll add a few notes to the "NPC card" and put them in the "recurring NPC" file and make a note to try to make them appear again. If they still get a good reaction, I'll escalate things again and move them up another category.
If you can get them to react strongly to an NPC in multiple settings, then you should do a full flesh-out of the NPC and start building up the structure behind them to make them become a major recurring villain.
