I don't think it's a given that he will be. With Gaim not performing well sales-wise (and Wizard also had more than two Riders, and didn't do brilliantly either) we're probably back to the two-Riders-only format for the foreseeable future.
My current guesses are:
- Something is "wrong" with Gou: he's either been somehow affected by a Roidmude, or the use of Mach is doing bad things to him, and he's slowly deteriorating. This is why he seems to be in a hurry to do something, why he is clearly keeping something from Kiriko, why he wants to hook her up with Shinnosuke (i.e. so there's someone to take care of her, etc.) Not sure at the moment whether they would kill him off. It seems rather depressing, and if he's popular with fans, they'll want to keep him alive ... on the other hand, there was that interview where Sanjo said that Ohmori wanted Drive to be a lone hero.
- The Roidmude unit fragmenting with Heart, Brain and Chase turning against each other. (Medic may betray Heart and side with Brain, but I'm hoping she will strike out on her own.) After she manipulated Brain into doing what she wanted in the last episode, I'm thinking she may become a bigger threat in the second arc of the show, but she may be killed off at the end of that arc - perhaps explaining why she isn't in the OP credits.
- Chase will be redeemed, but then sacrifice himself and die. He will not go over to Shinnosuke's side until near the end of his arc
- The show is getting through Roidmudes so slowly that it will be forced to rush through them at the end (like Fourze did with the Horoscopes, but worse, as there are way more Roidmudes.)
- There'll be more shitty "romance" nonsense with Kiriko and Rinna
- Kiriko and Gou may have some kind of unique body chemistry explaining why Kiriko couldn't become Drive and tying in with what's happening to Gou (like in W, where Terui was mysteriously immune to the Old Dopant.)