As the Sengoku Jidai campaign of the original release began in 1530, the Tokugawa clan did not yet exist; the family (Matsudaira, of Mikawa) were vassals to the Imagawa at that time. Tokugawa Ieyasu appears very late in the game as taisho of the Imagawa. Technically the clan name should change but I think that would have been awkward from a gameplay point of view.

In the Warlord Edition release, the Tokugawa do appear as a clan in the 1580 campaign (replacing Imagawa), with Ieyasu appearing again as a 6-star daimyo (there are additional campaigns you can play in addition to the Sengoku Jidai: 1530, 1550, and 1580). Honda Tadakatsu appears as one of his generals (in Hizen at the start of the game iirc).

I was surprised not to see the Chosokabe, Otomo and/or Ryuzoji as playable factions in the original STW. The Hosokawa, Asai and Asakura were also significant. I would enjoy seeing these included, though I would be satisfied to see them represented as individuals leading units, like they did in the original STW.