This here is a little idea I've had sitting in my head for a long time for an alternate ending to the Naruto series. Of course, I actually came up with this before the series had actually ended, but still. It's also sort of a crossover setup, if I ever get around to it.
Disclaimer: I Masashi Kishimoto owns Naruto.
A man sat, alone on a barren planet that had once been filled with life. There was little else to do at the moment, as he prepared to leave. Before that, however, he decided to spend his time remembering.
The Daiyonji Ninkai Taisen had ended with Ōtsutsuki Kaguya's defeat, but Uchiha Sasuke had not been satisfied and was determined to enact his revolution. Uzumaki Naruto stood as his only obstacle and the two had done battle.
They tore land, air, and sea asunder as they clashed, a battle of wills unlike any the world had yet seen
In the end, Naruto stood in bittersweet victory, but Sasuke understood and, in his last act, transferred his eyes and power to Naruto, entrusting him with their dream for peace through different means.
Naruto, though upset, accepted his friend's gift for what it was. Now with the full power of the Rikudō Sennin, a pair of Rinne Sharingan easily as powerful together as the eye Kaguya had, and the power of all nine Bijū within him, Naruto was now immortal, able to live forever and guard the peace by his own means as Sasuke had desired to.
Carrying his deceased rival/brother's body back to where their teammates, Hatake Kakashi and Haruno Sakura remained, the former immobilized by the strain of using the Sharingan through even though he didn't even have the eyes for it and the latter by a powerful genjutsu from Sasuke.
As he arrived, near sunset, Sakura had already awakened.
The team spent a fair while mourning the passing of one of their members before Naruto declared his intention to dispel the Mugen Tsukuyomi entrapping the earth.
The release was a success, but something had gone wrong.
Kaguya, pressed heavily in her battle against Naruto and Sasuke, had pulled heavily on the chakra of her victims and the planet to bolster her strength. Anyone with too little chakra had already been partially turned into Zetsu. Considering that the majority of the Shinobi alliance had already been critically low on chakra, only those there who's chakra could cope with the strain even while that low survived, cutting the already meagre twenty thousand survivors to a paltry seven hundred.
Among the dead was Hyūga Hinata, whom Naruto mourned especially heavily, knowing that there had been something precious forming between them.
With Naruto's immense sensory capabilities, this main group of survivors left the decimated battlefield to search for and gather any other survivors, accepting anyone and everyone no matter their former alignment.
Things such as 'Village' or 'Country' hardly mattered anymore.
Eventually, their numbers swelled to nearly two thousand, but there was an even worse outcome of the war. The Mugen Tsukuyomi had entrapped not only humans, but wildlife as well and had not spared them death either.
Quite simply, there was no longer any game or cattle and the survivors were forced to survive off of what they could grow from the ground.
Eventually, they settled in a central area of the continent, sending out search and gather parties to scavenge supplies from the ruins of the world around them.
Eventually, another effect of Kaguya's greed became known as the plantlife began to die as well. She had truly drained the entire planet to near catastrophic levels and now they were paying the price.
Making a decision, the survivors turned to Naruto and named him their legacy, the legacy of the shinobi.
They taught him everything. Everything they couldn't teach him, they found documentation of in the former Hidden Villages. Then, in the end, as their food supplies dwindled and sickness abounded, they gave to him all of their chakra, leaving him alone again for the second time in his life, but with a purpose.
Even the Bijū were gone now, giving him their chakra and turning him into the new body of the Shinju as they told him that, as they were born in the old Shinju with the planet, so too would they die with the planet, creating the new Shinju.
So charged, Naruto combed the planet and gathered all knowledge there was to be had. From the former Villages, from libraries, from summon realms, from civilian and shinobi sources alike, Naruto collected and learned or stored everything there was to be had of his dying world before finishing the job, absorbing the last of the planets lifeforce into himself.
He spent another decade training before he deemed himself ready.
Standing on the barren surface of his planet, he summoned forth the massive sphere of earth containing Kaguya and shattered it, completely releasing her.
Black Zetsu was dead before he even managed to get his bearings and Kaguya was ruthlessly crushed, unable to compete now that she was down to nearly half-strength and her opponent was both far more skilled than her and her equal in power.
Naruto claimed the origin of all chakra as his and, though not quite as the deceased Rabbit Princess desired, all chakra was one again.
Naruto then worked diligently to master his new power, awakening a new eye in place of Kaguya's Rinne Sharingan, a Rinnegan with the Byakukan fused to it he deemed, fairly simply, the Rinne Byakugan.
In this eye, he found an amazing power. With focus, the eye could view back in time.
Flying high above the world, he reflected his eye off of the moon and saw everything. Everything that had ever occurred on the surface of his planet was his to know.
Naruto put this knowledge to good use, learning long-dead arts and ancient techniques he'd not yet seen and never been taught.
Eventually, what could have been centuries after the death of the world, Naruto was satisfied that he had mastered everything, that he was truly the legacy of his people. And so, he decided to move on.
The man stood, finally ready after having slowly – unnecessarily so – built his chakra over the last few hours as his mind wandered.
In an instant, he was far from his homeworld, facing it was a marble in the distance, facing it from its own satellite. With a gesture, the once vibrant sphere disappeared into another dimension. He left the moon behind, it was not a natural satellite and brought up too many bad memories. He went to the four other planets near his own and, though they bore no life and no interest to him, he drained them of their chakra as well, before storing them away.
Then he turned to his final task.
He'd never realized how beautiful their sun was. Belatedly, he realized as he dipped his hands into its seething, roiling, turbulent, and overall incredibly hot surface, that he was pretty sure no shinobi, not even Kaguya, had ever imagined being so powerful has he was now.
Slowly, over the course of many days, the star cooled and shrunk until, finally, it was little more than a cloud of hydrogen and helium gas which he then sent into storage as well.
Turning to the multitude of stars in the vast, empty expanse around him, a trio of eyes sparkled in anticipation of what might be out there.
Space folded to his will and he shot forth, moving faster than light without ever really moving at all, eager for what he would see.
Uzumaki Izō Naruto was now loose in the universe, ready to offer his help and share his ideologies with all who wished to hear, and even those that didn't. He had a legacy to preserve, afterall.
Izō means legacy. Naruto decided to put it in his name as a permanent reminder.