Here's some interesting way's to make an old story fresh for a DM story prompt.

these aren't necessary full campaign or one shot ideas. it's more of a the DM can take this idea and make something with it. I'm also someone who likes the sidekick rules so there's no DMPC issues. rather NPCS you as a DM can use in a sidekick roll.

The Classic: the knight saves the princess and slays the dragon. It's what it says on the tin. 

The Roll Reveal: princess comes to save her knight and slays the dragon. Like Classic but with a minor change.

Gender bent: the lady knight comes to save her prince. And slay the dragon. Possible love triangle if the dragon is kept alive. 

Evil Princess: the knight is here to save his dragon partner from an evil sorcerer's princess. He slays the princess and saves the dragon. This is a twist most don't see coming. 

Dragon Parent: a dragon adopts an orphan little girl unknowingly taking in a princess who ran away from her evil family who wants her back as a blood sacrifice. With knights breaking into the dragon's home trying to slay them and kidnap their child. The dragon stays the knights and protects their princess. It was self defense your honor. 

The Giga Chad: the knight saves both the princess and the dragon because he's not a speciesist and wants to save the people who can't protect themselves. It's clearly the king who's the bad guy here. enslaving this dragon and putting his daughter in a tower like a prize to be won. The knight slays the king and brings order to the realm and lets the princess take over as queen. He then rides off into the sunset with no prize except for the self-fulfillment of saving those who need his help. 100% based. 10/10. No notes. Absolute Cinema. 

The Siblings In Bonds: all are captured by the evil knight. Including a princess and a dragon. All in this place are brothers and sisters in binds. Then the princess and the dragon slay the knight and save their new found friends. It's a jail break story with extra steps. 

The Shrek Way: knight goes to save princess but is chewed by the princess for not slaying the dragon first but it's on his to-do list. The dragon and his best friend actually have a lot in common and fall in love. It's not ogre till it's ogre. 

The draconic love way: the dragon and the princess fall in love and they have a tussle with the knight before he becomes their supportive but not very bright jock himbo friend. Very wholesome. 100% Kronk vibes.