I can't remember who originally came up with the idea but I really like the idea of dragon born as a manifestation of sin. Basically a person who grows too greedy, too wrathful, too prideful will slowly start to notice that their teeth have sharped and that their skin has grown harsh, Eventually their skin will turn to scales and they will grow in size until eventually they became nether man nor dragon. Perhaps eventually as they indulge in their violent and selfish urges they will loose their mind as they continue to to grow in size. eventually they emerge as a true dragon.

Edit: Of course you would need an explanation for why every villain isn't a dragon born. perhaps it could have a prerequisite, maybe it's a rare cures or a side effect of steeling dragons gold?

Since they aren't actually related to dragons (did I read that right?), you might look at Lizardfolk and Yuan-Ti for inspiration.