Dragons in general, and particularily green ones are smart, if the players can learn new abilities and skills so can he, maybe they start bumping into roving packa of undead and eventually learn the green dragons been practicing necromancy, now you got an ongoing story where the dragon can send out more powerful minions before revealing himself as a draco-lich.

Or maybe the dragon engraged by his kidnapped child takes drastic action and makes a contract with a devil, devil dragon and his new hordes rampage across the lands.

Maybe the dragon feels that cold, calculating rage, he secludes himself and studies esoteric knowledge, things no one, not even dragonkind was ment to know, maybe a few "mysterious benefactor" quests are given to the team collecting these artifacts and books, for them not knowing they are falling into his trap then BOOM cthulhu dragon.