Interesting, and with you as the writer we can definitely look forward to a good story.
I do wonder how Sasuke will react to Naruto's departure. Because the way I see it, with that he's basically lost the entire reason he wanted to be Hokage. Sasuke is selfish, I can't imagine he actually wants to spend his days locked up in his office filling out paperwork, even in wartimes he'll be the last to enter the battlefield out of fear of losing him, and considering the number of deals he had to make to get the position, the amount of power he has has been restricted because he can't alienate the Daimyo or the Clan-heands too much. Basically I personally see Sasuke's interest in that title fading fast once he hears Naruto has essentially abandoned Konoha. That's not to say Sasuke can't cause some serious damage before he loses his patience and bails Konoha again.
As for your musings on the immortality-debate, I do agree that overpopulation would be an eventual problem. While I can't speak for others, the problem I had with the system was that it was geared heavily against the women of Naruto's harem. For starters it's biologically impossible for Naruto to give birth, as such the choice between immortality and a child is not one he personally ever has to make. Also, the no-child-condition is not going against any of his instincts. As a male, Naruto's biological imperative is to mate, as often and with as many females as possible. And he can do that, it in no way goes against the rules he has to follow to keep being immortal. The female's biological imperative however is to give birth to the next generation, that is in conflict with the rules to stay immortal. Not to mention that while society might order otherwise, the male need not be present at birth, or even aware of the child's existence, which coupled with the fact that as a male there is always the chance that the child's not yours, tends to lead to the bonds between father and child usually being not as strong as with the mothers, who carry the children to term, give birth to them and still them, which creates an intimacy a male is difficult to match. I disliked the conditions for immortality because they were basically geared towards Naruto having to pay as little of the tab as possible. Say a woman decides she wants a child. Naruto will obviously be the father, and the two become parents. The mother gave birth, and is stripped of her immortality, so she grows old and dies in return for being a mother. Naruto on the other hand stays young and immortal, the only price he pays for experiencing fatherhood is that he has to live with the fact that one of his women chose to essentially sacrifice her life to have a child. That's it. The system basically seemed geared to let Naruto have all the fun he wants satisfying his instinctual urges, while punishing any woman who wants to fulfill hers.
You once responded to the suggestion of simply having the children being mortal that it wouldn't be fair to them. I know it's a bit late, but I'd still like to say: "And?" Life's not fair kid, the immortality system was not designed for immortal children, it was designed solely so that Naruto's bedmates don't die on him. Tough luck kid, you're not a bedmate so you don't get immortality. No club-priviliges if you aren't a member, that's life. There is no such thing as 'fairness' when immortality is added to the equation, the very concept of immortality is unfair as long as it's not given to everybody, which as you correctly pointed out eventually uses up all the resources. So the argument of fairness isn't really a factor in something like this.
...And again I drifted completely off-topic. I'll just end it here before I go even further off the rails. All I'll say is nice first chapter, hoping to see more soon.
Author's response
Thanks for the review.
To respond to your statement, well to be honest you seem to be making two contradictory points. On one hand you are saying tough luck to the kid born of such a union as life isn't fair, while at the same time saying its unfair to the women of Naruto's harem in Eroninja.
Keep in mind that it was Kiyomi who put the system in place, and as such it is meant to heavily favor Naruto, and her. After all, she was already immortal to begin with so she made Naruto immortal as well. She did so for the other women since she knew it would make Naruto sad to watch them grow old and die. But you could also say she did it to reinforce Naruto's lack of interest in having children in this story.
But as you said life isn't fair, so in order to prevent a future problem from arising she made it with conditions. Furthermore, the women have a choice, its far more unfair and dare I say cruel to foster the consequences of that choice onto an unborn child.
As to the biology of it, well people defy what their biology tells them all the time. Some people feel no need to have children, others feel no need to ever find a mate to settle down. For some not having a child is a conscious decision since it would impede the lifestyle that they wish for. Not to mention that's not even getting into the issues of Homosexuality and such, which if having a child was so hardwired in then it would make lesbisim an impossible choice for a woman to make.
For the women of Naruto's harem they are given a choice, live for an eternity with the man they come to love, or choose to become mortal to have the next generation. It is purposely weighted so that those who have to make it choose option A as that is what Kiyomi knows Naruto wants. You could say that for Naruto the one with the heaviest cost is them becoming Mortal as he would have to live with the loss. You say Naruto doesn't pay anything really, but I think that would be a hard pill for him to take, especially as I stated kids aren't something he's truly interested in.