It seems that Sekigahara was one of those battles like Hastings that could easily have gone either way, especially if Lord Hideaki had decided to side with Ishida Mitsunari and the Western clans instead of betraying them. So how would the alternative history play out if Tokugawa and his allies lost the battle?

I may be wrong but it seems that one of the reasons the Western clans were so thoroughly routed after the battle was that they had no easy way to retreat. As long as Tokugawa survived, it's quite possible that he would be able to fall back and regroup. Of course, if he was killed, I wonder if Ishida would be able to create a strong central shognuate, or would Japan face many more years of civil war?