Had an idea for Magearna that could be scary good. I dont know if there are already plans for her already but since she was made for royalty she could be in a abandoned castle with dark clouds over it. When the MC gets in he immediately finds her stone cold lifeless and missing her soul core in the middle. He hears a voice direct him to the dungeons below to find her. The ghost pokemon would try and stop him and if he fails at all they will move the core again while kicking the MC out for good. Once he has the core he could either battle the ghosts that possesses her body and need to tire it out to put her in or answer questions from her. She could explain that the ghost pokemon here are very friendly but when they get board like to do pranks on others. So the ghosts took her soul core out when she was in a stasis sleep and put her body in the main hall for others to find and laugh at the reactions and have fun with those that want to put her core back and have no intention of harming her or others.
Once she is back in her body unit she cant really move since it will take a few days to be able to move normally BUT there is a quicker way by making lots of stimulus in her body and is just able to open her legs for the MC to do what he does best! She would see that he has an extreme amount of pure love and good radiating from his soul that she can see that she wants to go with him.