I hadn't even considered that Battride War 2 would skip the secondary Riders. I'll be flabbergasted if they skimp out on that. I'll wait alongside everyone else and see how they handle it, but I'm definitely paying more attention to the game now, before it's released.
Eighting/Namco Bandai don't exactly have the best record when it comes to Rider games. Take for example the Climax Heroes games. It took them three years to finally get the roster up to a respectable size, and two years to finally give characters who had been playable since the first or second game complete movesets (Gatack, Kaixa, the Hoppers, etc). The first three CH games were just
dire. It wasn't until Fourze that it became a decent game, and Super that it became a legitimately good fighting game.
As far as Battride II goes, it wouldn't surprise me at all if they leave the secondary Riders as NPCs. Some of them don't fit in with the game's storyline (Kaixa doesn't appear in the part of Paradise Lost that Faiz's movie stage is based on), some of them don't have bikes, most of them don't have secondary forms (though they could just give them a generic "powered-up" mode like they did with Beast in the first game), etc. Of course these are all problems that could be worked out one way or the other. The biggest thing, I think, is that these games are just low-budget as shit. Look at some of the promotional material we've seen thus far. While there are several new stages, they're also clearly re-using several of the stages from the first game. We've already seen the city, forest, and mountains stages from the first game in trailers/screenshots for II. And if they really do skimp out on the secondary Riders, it's going to feel more like a half-hearted expansion pack rather than a true sequel. Y'know, kinda like Climax Heroes. And it certainly won't be worth paying full price for.
EDIT: You know, I just noticed something about that picture I posted. While there appear to be four open slots next to Baron, if you look closely you can see that the two furthest to the right are darker than the other two, seemingly implying that they won't be filled in by an unlockable character. That's...troublesome.