First, let me start by saying this is just pure speculation. Rumors surrounding new Suikoden games have popped up often, especially around TGS time, but usually with very little backing. However, a few developments have occured over the last month or so that all strongly point to Konami announcing a new Suikoden game, possibly at the upcoming Tokyo Game Show.
First, Konami recently trademarked the title Genso Suikoden, as discovered by the trademark slueths over at Siliconera.
Blue Moons Studio, the Japanese software studio founded by Suikoden creator Yoshitaka Murayama, announced they were working on a new, unannounced RPG for an undisclosed publisher.
Further, Murayama recently did an interview with Swedish magazine Level about his career with Konami, despelling rumors that he left the company unhappy after Suikoden III. When asked if he would ever rejoin the Suikoden team for another Suikoden game, he replied "If Konami would give me an offer to come back I would seriously consider it."
Finally, Konami released their TGS page earlier today, showing 5 unannounced titles set to debut at the show.
As I said, this is all pure speculation, so take this with a grain of salt. Tokyo Game Show is taking place later this month, from September 24-27.