Yes, I agree. I think Jean has to a) vanish from the earthly plane in a fiery Phoenix flash b) return to normal power levels. To have her appear to visibly, physically die again doesn't feel right at all. It would have to be a Phoenix firebird sacrifice, as was envisaged at the end of X2 before Sentinels were ruled out... a disappearance in a massive burst of energy with a gigantic firebird effect... This is a little like the original comicbook when Dark Phoenix launched into space with a huge firebird (witnessed by the Hellfire Club!), and leaves the door open for an 'X-Men in space' story (as Brett mentioned) if that's the way the franchise goes.
She could redeem herself - before the fiery flash - only by stopping/defeating Magneto and other Brotherhood members that haven't already been defeated.
Ironically, it was Magneto who twice defeated Phoenix in the comics (once he siphoned off her cosmic lifeforce in a 'magnetic bottle' - a containment system used to control plasma energy.. plasma being the energy of the sun and stars; a second time, he gave her a lethal stoke by channelling the planet's electromagnetic energies into her; both times she was caught offguard).
But for movie purposes, Jean defeating Magneto would work well as a redemption, especially if she has killed Xavier and even Cyclops earlier on. I rather like the idea of her reducing him to infancy (as happened to him in the comics when a mutant he created turned on him)... but that may be a bit 'far out' for the movies.
If she were to be returne to normal, rather than the fiery disappearance, defeating Magneto in a supreme effort of her power could burn out her excess power (burn out that part of her brain) and return her to normal in a sort of naked, amnesiac confusion. Not easy to work with the character after that though... hard to have her sashay back into the school after all that has happened, life would never be the same...how would X-Men and students be with someone who had destroyed the professor, sided with the enemy. Which is why i think a fiery goodbye from the earthly plane works best.
This leaves Magneto's children (at least Wanda and Pietro, if not Polaris) to plot revenge on the X-Men in a future plot (they could join, or create, a new Brotherhood), and the cure story and the deaths of several leading mutants (including Xavier) is a good trigger for Sinister/Apocalypse/Hellfire to spring into action on protecting/enhancing the gene pool. Bring in Gambit and Bishop as good mutants, Emma Frost as the double agent perhaps, secretly working for the Hellfire Club.