It could be Orochimaru. He's always been after powerful ninja tools and the Shusui Sword would be no exception. He might want to use it to enhance his own jutsu or gain more power in the ninja world.
One idea could be a story where Naruto discovers the Shusui Sword in an ancient hidden temple. He has to learn how to master its power, which is tied to an old ninja clan's secrets. Maybe he has to face off against a new enemy who also wants the sword for its power.
I recommend two novels to you: " Stealing the Dragon Prince's Only Pearl " and " Online Games Dragon Return to the Shallow Abyss." " Stealing the Dragon Prince's Only Orb " told the story of a human stealing the Dragon Prince's Divine Power Orb, causing the Dragon Prince to degenerate into a small dragon, and the human being was hunted down by the Sea Tribe. The main character of " The Dragon of Online Games Return to the Shallow Abyss " was the young master. After stealing the dragon egg, he was chased by the divine dragon and was forced to embark on an adventure. Both novels fit the plot you described where the protagonist stole the dragon core.
Fan Xian successfully obtained the key by breaking into the empress dowager's bedroom at night and stealing the key. He used the key to open the box left by Ye Qingmei and found a sniper rifle and two letters inside. Fan Xian had chosen to barge into the empress dowager's chambers at night because he knew the key was in the empress dowager's hands. Fan Xian did not return the key. Instead, he made a fake key and put it back in its original place to avoid arousing the empress dowager's suspicion. Thus, Fan Xian successfully stole the key.
Well, it could be a rival seeking power or someone with a personal vendetta against the empress. There are several possible suspects, and the author might keep us guessing until the end.