1: If the summonings last forever and are as powerful as the real ones were, then Madara wouldn't need Sasuke to summond the Gendo Masou(or something), he could only use Pain again.
2: Maybe Hidan already has given his soul to Harashin in order to become immortal. So maybe that's why he cannot be summoned and not because he's alive.
3: But all in all, i don't think that this will result in Akatsuki all over again. Something will probably happend or there will be a catch, as someone said.
4: About that jutsu. If its not a new unknown jutsu that Minato left behind, then i believe that its the death reaper seal jutsu.
5: First of all, how do you learn such a tech if you only can use it once? I mean, how do you train with it?
6: Second of all, can you over-power the death god if you're strong enought? Maybe with the kyuubi's help, Naruto may learn to use it as much as he like without dieing.
7: And lastly, it could also be that Minato sealed the fox in Naruto so that he can use the tech one time without dieing, but instead the fox will be taken as someone said. In a final fight with Madara for example.
8: One last thing. Is the only Sages in Naruto Toad Sages? If so, does that mean the Sage of the six paths was a toad sage, making the toads one of the first summonings?
9: Maybe he had a unfinished jutsu that he passed down the line, but none of the succsesors was able to perfect it? Maybe Naruto will manage to do it? that would be awsome to see
Any thoughts?