You know, that is an interesting question about the next movie. What can it top this movie? *cough* NaruSaku romance *cough* I think they will bring Kishi back to do another movie. It's an assumption but it's possible. I mean about now, it will be hard to go back to the usual routine of a movie. So, now what? I mean look at One Piece. When Oda did movie 10, the next movie was something...too different, staring with cgi. Now he's back again for the new movie. So, it's possible for Kishi to return. And bring more NaruSaku! Hahahahahaha!
Man, it's still funny that we rose to the top after the movie released. I mean look at all these new fan arts. This forum is more alive than ever. And the manga is reaching to the point where Naruto and Sakura will meet again. This is going to be great.
I don't think he'll come back to the next movie as the guy in charge, but maybe as a writer or the person who comes up with the story concept. I don't think he'll come back because this was a lot of work for him, and he was still doing the manga while also doing the movie. But then again I didn't know that the author of One Piece is doing an other movie with him in charge, nor did I thought that the author of Dragonball will come back and be in charge of the upcoming movie, despite it being YEARS since the manga/anime ended and when he hardly (if any) had any input on the anime when it was around.
As for future movies, let see how many there will be in the future. Let see, the manga will probably not end until 2014, and the anime will probably last longer than that. At the least anime will last about a little bit less of a year after the manga ends, but let's say it lasts up to 2016, provided that they don't decide to continue the anime in a new series (like Dragonball GT). Since they like to release a new Naruto movie once every year, they may still make four new Naruto movies, yet again provided that they don't continue the anime or that don't take a break in one year.
Here's my ideas of what should be about:
Seeing that Minato and Kushina are very popular (and the promise of them being in the movie leads to more ticket sells), why not just have a movie about them. It will of course take place in the past (or unless they come back as zombies, or edo ninjas), and will give more details about their past and life. I could imagine Kishi writing the outline, or possibly the whole script, for this story, and if he really wants to, be in charge again. Admittedly, the anime studio seem to want the movies to be about Naruto, as he is the title character, so they may want a movie more about him. But then again, a story about his parents, who are really popular, is giving his character more of a back round and history, so they may go with that still.
An other possibility is that rumored Sasuke story that Kishi want to originally do for the movie, but decided to do Road to Ninja. I don't know what kind of idea it he had in mind, or even if he already had a story in mind, but I'm still, curious of how a movie starring Sasuke will turn out.
This next idea I see more of happening once the main anime is finished, or in an other upcoming milestone. I'm thinking of a two part movie that recaps the events of the entire story, just like those recent flashback episode that retold the entire story of Part 1...........but the twist is that the story is told differently. Like the Dragonball anniversary in the 1990s, where that just told the events of Dragonball but it was drastically different. It will be like a different universe, retelling of a story that we already know but with it's own twist. I see them splitting these movies into two, one about Part 1 and an other about Part 2. That's of course if they decide to do anything like that at all.
My next theory is probably the most likely of them all, the studio just keeps doing what it has be doing, coming up with original stories that take place the Naruto world but are not in cannon with the main series.