We all know about Jiraiya, Orochimaru, and Tsunade being the legendary three Sannin. My theory is simple and probably been put forth before, but I have a bit of a twist.
Naruto, Sasuke, and Sakura will be the next generation of Sannin. However, they will not be called "Sannin," they will have another name given them that will portray their high ranking skill as shinobi. The manga will play itself out so that the three of them will come to a place where they each have the same goal so that they will be fighting together. It is in this time that they will face an extremely strong shinobi such as what Jiraiya, Orochimaru and Tsunade faced.
I believe this shinobi will be Pein. After the current attack on Konoha comes to an end and Tsunade has died, the three members of team 7 will cross paths for the last time as allies against Pein. It is here that Pein will not be defeated but his secret will become known by Naruto, Sasuke, and Sakura so they will be able to fight him without dieing, but at the same time will not be quite strong enough to fully defeat him. We've all seen how each of them has grown in the same way of their sensei. All that is left is for the three to have that defining moment that will make their names known throughout the entire shinobi world just as Jiraiya, Orochimaru, and Tsunade. Pein will give them a name just as Salamander gave the name Sannin. At this point, Sasuke will go his own way once again, just as Orochimaru did, and Naruto and Sakura will return to Konoha.