I like the "unscripted villain" myself. Just
Just keep notes on anany surviving npcsNPCs from adventures you have been runningyou've run the PCs through and see which ones your characters reactreacted strongly to - usuallyto—usually I keep a notepad file on npcs and ifNPCs. 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 thethem appear again. FIf they still get a good reaction, I'll escalate things again and move them up another category; ifcategory.
If you can get them to react strongly to an NPC in multiple settings, ... Thenthen you should do a full flesh out-out of the NPC and start building up the structure behind them to make them become a major recurring villain.