The one huge thing which imo CANNOT be skimped on is the animation. If everything is stiff and dodgy in its animation that would really REALLY be a let down.
Nothing more needs to be said about this. We demand sexy animation.
But something needs to be said about THIS...
Dragons.
We demand dragons.
Giant, hulking, flying, vicious, ravaging dragons. They don't have to be as big as the Ancient Mountain Dragon in Monster Hunter, but they need to at least be able to be at least twice as big as a Rathalos. And they need to breathe fire. Acidic streams of fire, and they need to shoot fireballs too, okay not really but it could be a nice surprise attack if you never really expect the fireball to come out scream toward you.
It would have to be a massive challenge (optional quest, you'll see in a moment) just to GET to the dragon on it's own island that it guards against tresspassers (maybe the island has precious minerals that a nearby area needs or herbs to make a medicine for any type of disease in that region), then if you decide to attack it you have to kill it. Completely kill it. And if you don't have the proper equipment and items to kill it COMPLETELY, it'll just stay there dead for a while, and you can continue your quests. Notice that the dragon, while alive won't leave its island...
But at some point, if you didn't kill the dragon properly, it comes back as a vengeful Zombie Dragon, more powerful than ever before, and it breathes a cold blue wraithful fire that converts all it touches into its nigh-invincible servants who's sole purpose is to help the dragon kill the one who would dare try to kill him. Of course this means the dragon does indeed leave its island and is going randomly from town to town burning all it sees with its unburning flame of revenge.
So you'd have to hunt all over the world and kill it again to stop the curse and so you can continue quests (they won't give you quests if they're busy trying to kill your for their serpentine lord), but you'd be in for the fight of your life because if it was hard as hell to kill before, you'd better prepare yourself for THIS.
It's a totally optional quest of course; you could always ignore the plea and choose another way to get what the area needs. And a smart person or an adventurer who didn't have the physical might to go toe to toe with that behemoth would find a way to use the island it somehow can't leave to unsummon it (how do you think it got there in the first place? An unsung war eons ago perhaps?). But you wouldn't get the awesome 'legendary' weapons embedded in its hide from countless other adventurers trying to kill it off if you were to just make it disappear.
Choices...
In short, a dragon would rock, and if you were to somehow implement this... dragon quest... of sorts, it would be too sweet. Come on... DRAGONS.
