I agree, I think Phoenix is the only codename that makes sense to her. Marvel Girl is so outdated and infantilizing. I'm wondering if Rachel's new codename Prestige will stick? I doubt it.
People know her widely by just "Jean Grey". The movies have also popularized her with this name and Logan and Cyclops shouting it. So why even add a new codename?
I would like something a little cosmic like "Supernova" or maybe just "Redd" from her days living in the future?! I think it has a nice ring to it.