I wouldnt remove Imagawa just because of Tokugawa.. You could put Tokugawa in campaign as Imagawa's and Oda's ally.. But Tokugawa would have much better realitons with Oda than Imagawa... So Tokugawa will choose to stay on Oda's side if Imagawa and Oda will have a war. I dont even know if this is possible.. But thats what i think.