Five pokemon I want to see:
Feraligatr - It's vicious and powerful. It looks somewhat mean
Arracks: Ice Punch, Hydro Cannon, Aqua Tail, Superpower
Tyranitar - I wouldn't be surprised if this guy had a pseudo-legendary. A dark type would fit his persona. It's Rock typing would also add nice coverage.
Attacks: Stone Edge, Dragon Dance, Hyper Beam, Crunch
Tangrowth - It's mysterious looking. A serious natured Tangrowth would be cool.
Attacks: Power Whip, Energy Ball, Sleep Powder, Poison Jab
Magnezone - Adding two types to Takuto's arsenal. Magnezone seems to fit the stereotype of going to a menacing, powerhouse trainer.
Attacks: Gyro Ball, Lock-On, Zap Cannon, Flash Cannon
Houndoom - Well it's just badass. I'd like Takuto to be a Dark specialists. Having mostly Dark types or dark-looking pokemon. Ash has never battled one before.
Attacks: Overheat, Shadow Ball, Fire Fang, Beat Up
And
Darkrai - Dark Void, Dream Eater, Dark Pulse, Roar of Time