Priorities:
1.) gunpowder tech: even though the three free promos don't mesh well, it's still three free promos, and practically the prereqs for all other promotions
2.)civil service/machinery: barring praets, I don't think there's a single unit in the game that can run the field in their era as well as samarai. If you can pull it off, an oracle sling for machinery is nice, and the oracle's priest may be able to bulb most of CS.
3.) Unfortunately, lots and lots of cottage cities. Since losing the organized trait, Tokugawa is perhaps the best warmongerer, but is practically helpless in acquiring the techs/infrastructure necessary to utilize this gift.
3.a) To keep ahead technologically, don't trade techs. Rather, start a war, raze a few cities, flex your muscles, and demand techs for peace. Along with Julius, playing as Tokugawa means that you should feel that other civs PAY to have a state of peace with you.
4.) kind of an afterthought: get ecology. Shale plants clearly answer the question about what power source to use for factories, and the answer is to have lots of polluting buildings, 1 from forge, 1 from factories, 2 from shale plants, coastal cities have cheap drydocks for another, and the optional 1 from airports (though, as aforementioned, in addition to mobility he could really use the trade route) that's a gain of 6 health from recycling centers.