If you don't want to have to conquer the entire map, isn't it possible to choose to play to the last surviving clan- though this wouldn't help your current game?

If you want more of a challenge from the enemy clans, my modified campaigns have different AI invasion and build strategies. They also alter the unit stats so that peasant units have greater numbers (you could disable this if you prefer the original, but it adds to the challenge). There are two new campaigns- the original campaigns remain playable, but the start dates of the new ones were chosen so that the better AI daimyos appear early.

Download here: Shogun Hiryu