Alright, a collapsed town probably has a lot of goods left behind so theres your classic goblins, rotten food could attract troglodytes who can stomach it, living underground is something insectoids or kobolds would do, and the plains are often inhabited by gnolls.
Collapsed town, underground, plenty of loot, and once had a touch of civilization. I would say a kobold warren would be awesome. Like dragons they covet the goods of mortals, gold and shinies. Unlike dragons they are amazing at digging out warrens.
When using kobolds combat is like fantasy Vietnam. You have murder holes, deadfall traps, oil slicks to light on fire, and plenty of guirella combat tactics. They often worship dragons and will carve iconography of them. They fight dirty, often deploying animals and pests. Like bee hives, badgers, giant weasels, or angry skunks.