More Information That Wikipedia Left Out ...
The unused opening showed a stylized shot of Noa and Zagi back to back in a sort of reverse face-off. To say they were going to team up is like saying Faust, Mephisto, and every form of Nexus were going to form a sentai-type fighting unit simply because they all shared a similar sequence in Nexus' second opening. I don't know exactly how Lucifer figured into the Nexus plot but as far as I know, the story would still have had the Noa VS Zagi conflict at its core given that this conflict was introduced at the very beginning of the Ultra N Project - before Nexus had even aired!
My guess is that Lucifer is merely a concept that eventually morphed into Ezmael, the final monster in the series. Honestly though, the concept of Zagi as an imperfect, evil, chaotic clone of Noa created by the Visitors out of admiration is probably cooler than anything Lucifer could have been.
That and I'm still certain that Nagi would NOT have become Noa.
Here's Lucifer's concept art, by the way:
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Also, here are the images from the unused opening shots in question:
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Given the HEAVILY stylized nature of the openings, I've NEVER interpreted these images as showing a possible team-up between Zagi and Noa. They just look cool. And actually, the shots of Noa and Zagi together are from unused eye-catches, not openings.
Who were these "Visitors"? Why was this shown at the very beginning of The N Project - yet not developed during the story until it was, literally, too late?
And, what makes you so sure that Nagi would not have become Noa?
The reason I ask is that these are patented Hasegawa beats:
1) Good and Evil must stand together to protect Life itself.
a) Keigo Masakai, who brutally attacked Daigo Mandoka and stole his Spark Lens to morph into evil Tiga - ultimately tried to help the stricken Tiga at the end of the show, during Ghantazoa's attack.
b) General Gondou, who had brutally ambushed and
murdered Shin Asuka - ended up forcing Ryo Yumimura to execute him in order to revitalize Dyna during the beginning of the final battle with the Spheres.
c) Hiro Fujimiya, who assaulted Gamu Takayama and XIG countless times, ended up
surrendering his power to Gamu and sacrificing himself - after realizing that he'd been a pawn of evil and had brought disaster to the world. He was far more fortunate than most - he somehow survived, and got a chance to redeem himself, finally standing as a true partner to Gaia.
d) In the final Cosmos movie, Ultraman Justice, who had previously assisted Cosmos in the defense of Earth (probably protecting the last survivors of the race he'd saved from annihilation and had settled secretly, but peacefully, on Earth), was apparently brainwashed to attack Cosmos and humanity. But, he realized it was a pyrrhic victory and almost died defending Earth while Mushashi's family & friends revived him with their love and life energies. Then, teaming up with the revived Cosmos to help the NeoEYES take down the invaders - and settling on Earth to protect it while Cosmos accompanied Mushashi on the revived Noah Project.
e) In Mebius' battle with Emperor, at least one sworn enemy of Mebius ends up sacrificing his life to defend the weakened Mirai Hibino and GUYS, plus when Ryu (temporarily) inherits the Hikari Morpher, which had belonged to a sometime antagonist of Mebius, Mebius ends up with his own morpher, plus a morpher that had been redeemed from evil.
f) In Mega Monster Battle NEO, Reionyx Hunter Dail, who destroyed an entire space station to destroy Rei, killing who knows how many people; turned around and gave the human crew of the Space Pendragon a super-weapon. Then sacrificed himself to save the crew from his commanding officer, who had taken them hostage to force Rei to fight to the death. One suspects that since they'd come from the future, that they knew the Pendragon's crew would be instrumental in saving Ultra, and thereby, the Universe, from Belial.
As far as the angry Nagi not being able to hold onto the Light, well, in Mega Monster Battle, Commander Haruna, who had been deeply suspicious of Rei, finally came around and accepted him after he proved his sincerity. If Nagi could accept a murdering monster as a partner against a far worse monster, perhaps she would've been forced to give up her hatred as well.