Or maybe a scorpion king AU (the movie). Where Marco is one of the best bounty hunters in the business, and Ace is an oracle who is bound to king Sakazuki. Then the White Beard pirates who are rebelling agenst the king hire Marco to kill Ace, who is the moral of Sakazuki's army. Ace foresees that Marco will try to kill him and informs the king. Who stages a trap. Ace helps Marco out of it because, lets face it, no one likes Sakazuki and Ace thinks Marco will help him. And Marco comes back to kill the king, and kidnapped Ace.