Dragons
Not sure how well that goes with the lore.
For one, the events that we have are not against dragons themselves, but their minions. Claw of Jormag is a lieutenant to Jormag, Shatterer is a lieutenant to Kralkatorrik and Tequatl the Sunless is a lieutenant to Zhaitan. The only actual Dragon that you kill is Zhaitan in Arah.
For two, the dragons already have humane elements to them. For example, there’s that champion of Kralkatorrik who betrayed his master.
For three, the dragons have been described as something akin to natural disasters. They’re all out to destroy all living things.
So what you’re asking for is some evil thing that is even greater a threat than the Elder Dragons, which are already very much capable of ending all life on the planet. And honestly, if you can already end all life, the next thing up in the scale of power is destruction of the whole planet. And I don’t think we wanna go there, that’s more of a sci-fi thing.
Okay, you are right Olba the dragons we fight are champions of the Elder Dragons not the Elder Dragons themselves. The dragon that betrayed her master was Glint, and she was freed from her master by the Forgotten. The Elder Dragons have thoughts and thought processes on a much higher scale than any individual being in Tyria. They aren’t mindless beasts that are running around killing (those are the minions/champions, and they are mindless because they obey only the rules of their master, the Elder Dragon). The Dragons would never ally with the races against a larger threat anymore than humanity would ally with ants in order to stop the world annihilation. Sure ants could kill a human (with the right type of bite or with enough numbers), but that wouldn’t make humans see them as a viable ally against and extraterrestrial threat.