Thanks guys for your advice. I usually don't use too much Shinobi or BNs. I will try to do that.

Vanya, I won by taking Owari and extending eastward from there as you said, but I found this a bit too easy. My main problem for expending eastward was first Takeda, whose army is far superior to Imagawa's in 1580. The problem too is that behind Takeda, there is the Uesugi who crushes Hojo quite early in the game and becomes very powerful in this campaign. A bit later in the games it crushes Takeda. So if you take Shinano, you have to face in the beginning of the game a very powerful Takeda, and a bit later a horde of Uesugi. So expending eastward for Imagawa is not as easy as in the SJ campaign, and taking Shinano too early in the game is probably not an option, unless using Shinobis or BNs.