The world in which Suikoden Tierkreis takes place has a veriety of cultures and even a few non-human races. Acording to the games story almost all of the various cultures where a part of different worlds that had been absorbed into the One King's world. I also couldn't help but notice that the game did not seem to have any runes (True or otherwise).
This lead me to believe that just maybe the world that the main series takes place in was originally the One King's world. It would make sense considerring that the veriaty of non-human races alone, not to mention the amount of land is so great.
There has been mention of an ancient and mysterious civilization many times in the main series, that was spread over the entire world. If it was indeed the One King's world, than the civilization's colapse would obviously be caused by the One King's defeat.
This would of course make Suikoden Tierkreis a distant prequel if true.