In his tweets, we witnessed the evolution of the Riderman design from that being close to the original Riderman design to the one we saw in the film. In his notes, he emphasized on the sunglasses in the design and was told by Toei that GACKT will play Riderman in the movie. Of all the concept art that he illustrated, the one with the Cannon Arm was the one used in the film.
[Source: Keita Amemiya (2 | 3 | 4 | 5 | 6 | 7 | 8 | 9) via Kamen Rider FB]