you need an in-between Generational power make up and eternal power make up. The original senshi didnt' go from their starting powers to eternal so you should take that into consideration. look at the original power phrases and mimic those for your in-between powers. 

Suggestions for power up phrases 
Generational for Chibi Mercury
Neo/Generational Mercury Power for Sailor Mercury
Neo/Generational Mercury Star  Power for Super sailor mercury
Neo/Generational Eternal Mercury Star Power for Eternal sailor Mercury 
These are just suggestions for you you can play around with them and find what you like best for your character. 

You can even take some of her mother's attacks and sort of rename them to work for her, in addition to your own takes on her powers and abilities. 
If you have those power up phrases maybe look at coming up with attacks and other powers for those forms. 

Try adding more to her special abilities like she's good at sports or she's great a video games. She can be