This may not be significant to you, but to me, you're my first post since Approval! So a big moment for me :3
Mm..I find your RP idea quite interesting! Especially if the duality of not only Fire and Ice are in play, but also the difference in personality!
I personally find Elsa so much more enticing as a character than her sister. Mainly because she personifies a woman who has responsibilities and struggles. In a way she is more realistic because she needs to keep her emotions in check, and serve a greater good (much like many do in real life!). Elsa to me is cold and calculative. She is reticent and detached from others. But deep down, she cares for those around her in her own, personal way. She cares for her sister, but would be hesitant to show affection (as being a stoic queen is forever her image to many). She locks her emotions behind her frozen exterior, but at times it melts at her core.
Mm...just gathering ideas. I hope this draws your interest in some way. I would be very interested to learn about your OC :3
Hope to hear from you!