Withe the split-realm factions, I often abandon one side or the other. In the case of the Imagawa, It can also be fun to abandon central Japan for Kyushu, and play a Shimazu-like campaign as the Imagawa. Taking out the Shimazu isn't that hard, you get a No Dachi bonus province instead of an Archer bonus province, and enjoy the benefit of a better (but not necessarily more exciting ...) starting position.

If you like this kind of strategy, Try the 1580 campaign (STW WE), Tokugawa. Raze and abandon Hizen, and take Owari on the second turn. Sow seeds of terror and destruction with those nine BFN units. I love this campaign because you are basically forced to play "by ear," watching what develops patiently, and capitalizing on situations as they present themselves. In order to stay in the money, one usually has to chevauchee a lot. The BFNs do this wonderfully. I think the 1580 Imagawa campaign requires a more ... sophisticated (for the lack of a better word) strategy than the norm. And it's fun to play as Ieyasu.