Skull isn't on the poster? But just on the main drawing on the left-hand side?
The bonus has 50 Riders sticker book, but only 37 Riders are visible in that image. I guess that we'll get 50 playable Riders in the end, with the tv show ones being the ones in the picture, and the remaining 13 all being movie Riders. And there's a small chance of getting the next Rider as an extra again.
P.S. And yes - I think all of Ryuki Riders will be represented... once more.
It won't happen. They did that in Climax Heroes because they ported them from the Dragon Knight game. At most, we'll get Ryuuki, Knight, Femme and Ryuga, but I think it's more likely we'll end up just with Ryuuki, Knight and Ryuug.
If my guess about the 50 Riders based on the stickers is correct, I expect these will be the final 13:
Nadeshiko
Aqua
Skull
Eternal
Kiva-la
Arc
New Den-O
Gaoh
Caucasus
Kabuki
Glaive
Orga
Ryuga
-I don't think they'll add G4 because G3 doesn't seem to be there.
+there's a small chance for the next Rider as a last minute extra again.
Anyway, I won't be surprised by the movie Riders getting a large part of the line up because even in the original most of the secret stages seemed to be named after movies, so the staff of this series seemed to be interested in the movies even back then. Apparently OOO's movie form and New Den-O had some very limited data in the game too. The poster itself shows Skull, Eternal and Core, all movie characters.
There's also the monster facing Diend, which seems to resemble a redesign for Sadondas from Skyrider's movie (not the one from Megamax though). I might be mistaken about that one though.