Let me know if something like this would be up your alley: A young prince and his best friend (another noble or maybe a commoner) ride into battle together against an enemy kingdom/tribe/other threat. The prince only survives thanks to his friend's courage and watchful eye, so he vows to join their families together as a display of his gratitude. Fast forward 20 years, the king's eldest child has come of age, and the friend who saved his life is still unwed. The king had always imagined that their families would be united through their children, but must now fulfill his vow to his friend by betrothing his eldest heir to him.
There's a lot of potential for world building in the premise, and it can go into a modern or historical setting-- there are a lot of variables that are easy to manipulate. Would you be up for playing the king's friend?
I'd be up for chatting about something more ~forbidden~ or a different line idea if that doesn't float your boat!