I don't see why they haven't put in Dosu already, over the course of several years.

They can't say he'd be too hard, Dosu here has everything, a perfect argument. He has a basic, already encoded move set, they can't say the Sound Genin are too unimportant to add, because Zaku's here, filler ninja are here, and even Sakura's in the game.

I always found Dosu to be one of the more mysterious characters when I started watching the show, he has a really odd design, and the odd design characters are usually some of the best ones. And as Dosu mentioned before, Dosu wasn't going to let himself be a puppet.

Just make Dosu's dream come true, they more then deserve after all the effort they've put into for a character they love.