However, in 2000, Kamen Rider Kuuga aired. It was a really new take on the franchise, with the hero being an human instead of a cyborg, the enemies being a mysterious civilization and not the classic Shocker ersatz, the huge use of different forms (already started in Black RX, but which would be done in a more interesting way in Kuuga), and the choice of a realistic Tone. Kamen Rider Kuuga would be very successful, and the Kamen Rider franchise has started then a new era that still continues now, with Kamen Rider Wizard being the 14th show in a raw of that modern era of Kamen Rider. During that era, plenty of new concepts were used every year, while keeping the basic elements of the Rider franchise (except the notable case of Hibiki) (the belt, the "Henshin" scream, the insect design of the suit and the enemies being human sized monsters)
Let's see now who are the writer of each show of that modern era :
Naruhisa Arakawa (main writer) : 1-12, 15-16, 19-22, 25-26, 29-30, 31 (with Kiyoshi Takenaka), 33 (with Inoue), 34-49 (40 episodes)
Toshiki Inoue : 13-14, 18, 23-24, 27-28, 32, 33 (with Arakawa) (9 episodes)
Tsuyoshi Kida : 17 (with Kei Murayama) (1 episode)
Kei Murayama : 17 (with Kida) (1 episode)
Kiyoshi Takenaka : 31 (with Arakawa) (1 episode)
Toshiki Inoue (main writer) : All episodes except episode 28 (50 episodes)
Yasuko Kobayashi: episode 28
Yasuko Kobayashi (main writer) : 1-6, 11-14, 17-23, 26-28, 31-40, 45-50 (36 episodes)
Toshiki Inoue : 7-10, 15-16, 24-25, 29-30, 41-44 (14 episodes)
Kamen Rider 555 (Faiz)
All 50 episodes written by Toshiki Inoue
Shouji Imai (first head writer) : 1-3, 4 (with Soukichi Imai), 5-10, 13-16, 20-21, 26-28 (19 episodes)
Soukichi Imai : 4 (with Shouji Imai) (1 episode)
Junichi Miyashita : 11-12, 39-40 (4 episodes)
Toshiki Inoue : 17-19, 29-30, 43-44 (7 episodes)
Shou Aikawa (second head writer): 22-25, 31-38, 41-42, 45-49 (19 episodes)
a) Kida/ Takatera era
Tsuyoshi Kida (first head writer): 1-10 (with Shinji Ooishi), 11, 19-22 (15 episodes)
Shinji Ooishi : 1-10 (with Kida), 12-18, 23-29 (and involvement in 19-22) (24 episodes (+ 4 uncredited))
b) Inoue era
Toshiki Inoue (main writer) : 30-37, 40-48 (17 episodes)
Shouji Yonemura : 38-39 (2 episodes)
Shouji Yonemura (main writer) : 1-10, 13-14, 21-26, 31-38, 41-44, 47-49 (33 episodes)
Toshiki Inoue : 11-12, 15-20, 27-30, 39-40, 45-46 (16 episodes)
Yasuko Kobayashi (main writer): 1-10, 13-14, 17-49 (45 episodes)
Shouji Yonemura : 11-12, 15-16 (4 episodes)
Toshiki Inoue (main writer) : 1-16, 17-48 (46 episodes)
Shouji Yonemura : 17-18 (2 episodes)
Shou Aikawa (main writer): 1-7, 10-13 (11 episodes)
Yasuko Kobayashi : 14-15 , 24-25 (4 episodes)
Kenji Konuta : 16-17 (2 episodes)
Toshiki Inoue: 20-23 (4 episodes)
Shouji Yonemura (other main writer): 8-9, 18-19, 26-31 (10 episodes)
Riku Sanjo (main writer) : 1-6, 9-10, 15-16, 19-20, 23-24, 27-28, 31-32, 39-40, 45-49 (25 episodes)
Naruhisa Arakawa : 7-8 (2 episodes)
Keiichi Hasegawa : 11-14, 17-18, 21-22, 25-26, 29-30, 35-38, 41-44 (20 episodes)
Kazuki Najashima : 33-34 (2 episodes)
Yasuko Kobayashi (main writer) : 1-10, 13-20, 23-26, 29-32, 37-48 (38 episodes)
Shouji Yonemura : 11-12, 27-28 (4 episodes)
Nobuhiko Mouri : 21-22, 33-36 (6 episodes)
Kazuki Nakashima (main writer) : 1-6, 11-12, 17-18, 21-22, 27-28, 33-34, 41-42, 45-48 (22 episodes)
Riku Sanjo : 7-10, 13-16, 19-20, 25-26, 31-32, 37-38, 43-44 (18 episodes)
Keiichi Hasegawa : 23-24, 29-30, 35-36, 39-40 (8 episodes)
During the first ten years of the franchise (Kuuga-Decade), the most important writer was without a doubt Toshiki Inoue, who has been the main writer of three whole rider series (Agito, Faiz, and Kiva), in which he has written every episodes (except one of Agito and two of Kiva). Moreover, he took over Tsuyoshi Kida midway in Hibiki, writing every episodes starting with episode 30 (except 2). Lastly, Inoue has been a major secondary writer of all the others Rider shows until Decade (except Den-O), often writing more than 10 episodes of a show.
Another important writer is Yasuko Kobayashi, who has been head writer of three series (Ryuki, Den-O and OOO); Like Inoue, she likes writing most of the episodes of her shows as head writer, but in a less degree (while she's written 45 of the 49 episodes of Den-O, she only wrote 36 episodes of Ryuki out of 50, and 38 of OOO out of 48).
Kuuga's main writer has been Naruhisa Arakawa, but he pretty left the Rider franchise afterwards, only coming back for 2 episodes of W.
Another important writer has been Shouji Yonemura, who has been heavily involved in the Hibiki-OOO era, as head writer of Kabuto (where he wrote 2/3 of the episodes) and of the second half of decade, and secondary writer for Kobayashi in Den-O and OOO, and Inoue in Kiva.
Shou Aikawa has been involved in two shows as head writer: in the second half of Blade, and the frist half of Decade; the first part of Blade having been written mostly by Shouji Imai, who left halfway through the show.
Tsuyoshi Kida has been had writer of the first half of Hibiki (with Shinji Ooishi); he had previously written one episode of Kuuga and has come back as head writer of Kamen Rider Wizard (since Wizard isn't over, I didn't detailed the episodes writers)
Lastly W and Fourze, two shows having a lot of similarities have been mostly written by a team of three writers: Riku Sanjo, Kazuki Nakashima, and Keiichi Hasegawa, with Sanjo as head writer of W and Nakashima as head writer of Fourze; unlike Inoue and Kobayashi, the head writer only writes about half of the episodes of the show, leaving a good bunch of them for the secondary writers (mostly Hasegawa in W and Sanjo and Hasegawa in Fourze)
Another major point: it's likely that a lot of those shows have had a difficult production with at least three of them changing head writers midway through the show (Blade, Hibiki and Decade).
It's interesting to notice that shows like Faiz, Blade, Hibiki, Kabuto, Kiva and Decade are a lot targeted by criticism, and thise shows are either mostly written by Inoue (or Yonemura, who is seen as a Inoue follower) (Faiz, the second half of Hibiki, Kabuto, Kiva, Decade), or have had more than one main writer (Blade, Hibiki, Decade).
However, in shows without issues with head writers, the writing team has very few people (seldom more than 3, often 2 (Agito, Ryuki, Kabuto, Den-O, Kiva), and even in one case, only one (Inoue in Faiz).It can also be noticed that from Agito through Kiva, except Blade and the first half of Hibiki, all episodes of every show have been written by exactly three writers: Toshiki Inoue, Yasuko Kobayashi and Shouji Yonemura : it's pretty impressive for a 8-year long run.