It's possible to make them a small faction allied to Imagawa, where the player must build them from scratch. It's a bit historical though, to make Ieyasu based in Mikawa instead of in Imagawa as hostage.

Another possibility is to use the rebel faction system, where rebelling Imagawa would be the Tokugawa clan, although this may create the implication where other non-Mikawa province may revolt and create other "Tokugawa".