Here's a thought. What if the SO6P couldn't achieve true peace because there were powerful enemies in his way? If he did, then they could be resurrected and become the new villains. It's not to hard to believe that a higher level of villains would be introduced since we've already been introduced to reapers.
DBZ powerlevels would not ruin Naruto. Tactics can still be a part of it if everyone's buffed and their powers, although powerful and absolute, have weaknesses. This could allow for more strategy.
I agree. Finding another worthwhile conflict is doable if Kishi is truly a good writer.
I once heard a story about an author who was having an argument with someone about the importance of a good plot. The author strongly believed that the strength of a story doesn't matter if the writer is skilled enough. The other guy thought the opposite; he believed that no matter how good you are, a story based on a weak plot would be a train-wreck.
To end the argument, the author asked the other guy to think of the worst plot he could. He came up with a cross between pokemon and the roman legion (I can't really remember if it was the legion, but it was something roman... I think). The author used the idea to write what would later become a critically acclaimed, highly popular book. True story, by the way. I can't remember the author's name, but he has some type of connection to Brandon Sanderson.
What I'm trying to say is that on paper, a story may seem awful, but that is only a very basic shell of what it's actually about. You have to consider the whole package; interesting characters, humor, the journey from one plot point to the next, etc.
Edit- I did a quick google search. The author's name is Jim Butcher, and the book (which seems to actually be a series) is Codex Alera. Here's a quote from Wikipedia:
The inspiration for the series came from a bet Jim was challenged to by a member of the Delray Online Writer’s Workshop. The challenger bet that Jim could not write a good story based on a lame idea, and Jim countered that he could do it using two lame ideas of the challenger’s choosing. The “lame” ideas given were “Lost Roman Legion", and “Pokémon”.[1]