You're certainly welcome here.
I am neither a step parent, nor a stepchild. But I love the Pandora bracelet idea. Simple. Elegant. Room to grow... along with your relationship with her. It will grow with time. And as you figure her out (as Julie mentioned)...as you learn to "get" her... you can add charms that represent that.
You could even present it to her with the explanation that as you learn to understand her, to "get" her, over time, that you will be able to fill up her bracelet. But for now, it is the promise of the future. It's your promise to understand her. To value her.
Talk with your fiance about the best way and words that will be meaningful for your soon to be daughter. And follow through. Don't promise to fill it up and then fail to pay attention and recognize what is important in her life and not do anything else with it.
Personally... I think charms added later (that are thought out and appropriate) would be most meaningful being given to her out of the blue. No musical notes after a piano recital... you know? Just... listen to her practicing and one day, when she is wrapping up, tell her how much you enjoy hearing her play, what a gift she has (if she does), be aware of the joy she gets from playing, and THEN give her a music charm. (just as an example).
But that's just me.
Anyway,
Congratulations.