Yoshie Gotou, one of the Stage Moms in "The O Chain", actually appeared in Kamen Rider Kiva as the woman who went into sports archery after her own Stage Mom pushed her to learn the violin. Karma's the real bitch here.
At one point, Jinno notes that Takeshi would be a nice name. Also, his use of Gratuitous English mirrors his schtick as a Fake American. And the name "Catherine" pops up later... note Nadagi is part of the duo "Dylan & Catherine"
Masayuki Deai (Trigger Dopant), though not for one of his own characters. In W Returns: Eternal, he gives the classic BoukenRed "Good job !" and thumbs up.
In Begins Night (W's half of Movie Wars 2010) there's this hefty Museum Mook (Giant Mook?) who jumps and attempts a Rider Punch on Soukichi. He's played by Jiro Okamoto, suit actor for Kamen Rider BLACK.
The Fuutic Idol judge who wasn't a Creator Cameo by the opening's singers was played by Ichirou Mizuki, who sung many songs in the Showa era, including the openings for X, Stronger, and Skyrider.
Actor Existence Failure: Minoru Tanaka, who played the villain of the Kamen Rider Accel direct-to-video movie, committed suicide four days after the movie was released.
Casting Gag: The boy who Shotaro helps (and teaches the way of being hard-boiled) in the last episode is played by the same actor who played preteen Shotaro in the first episode flashback. He also plays Shotaro again in a small part of Movie War Core in the Skull segment.
Dawson Casting: Mitsuru Matsuoka was 40 when he played Katsumi, who appears to be in his early to mid-20s based off the information given. Possibly justified since storywise he was growing up using an artificial chemical.
Defictionalization: Queen and Elizabeth are played by two members of idol group AKB48, and when they released their joint single "Love♡Wars", the Liar Dopant arc had them appear on an American Idol-esque contest (Fuuuuuutic Idol) performing the song in order to win a major label release.
He Also Did: Watcherman's actor, comedian Tomoaki "Nasubi" Hamatsu, is generally best known for starring in Susunu! Denpa Shōnen, an early Reality TV show where he had to spend a year living off of prizes he won via mail-away sweepstakes contests.
In Memoriam: The S.I.C. Hero Saga novel Playback, which covers this series, is a tribute to Shotaro Ishinomori, featuring an Alternate Universe version of the franchise's creator being guided by Philip as he learns what has become of his work.
Name's the Same: There's a Nasca in the third Sgt. Frog movie. She's also an alterego (real name Miruru), with the ability to sprout wings.
And yeah. Saeko Sonozaki. Did she share a name with a certain Kamen Rider Faiz character and had a similar position for the villains?
Sonozaki you say? As an added bonus, Wakana was in a live-action adaptation thereof as Mion.
Sokichi Narumi, with the white hat and all, brings to mind a certain Sokichi Banba, better known as Big One of J.A.K.Q. Dengekitai.
Kamen Rider Blade also had a recurring villain named Isaka who was responsible for killing someone the red-colored secondary Rider loved.
From "The B Carried on the Wind", Isamu Bitou, much like Isamu Ozu, had been locked away from society for some time (albeit for different reasons). It's worth noting that CycloneJokerXtreme, who debuts here, carries a sword and shield that slide together, much like Wolzard's did. He also has the same family name as Kouta Bitou, who also is friends with a guy and a girl, though in this case all three are Just Friends (close friends, but still).
The client from episodes 45-46 is named Kyoko Todoroki.
Surprisingly, Shotaro didn't freak out at the fact that his partner's true name is RaitoSonozaki.
What Could Have Been: "The O Chain" two-parter reveals that Shroud's ultimate endgame for having Ryu partner with Philip would be CycloneAccel Xtreme, the ultimate weapon in her war against Ryubee. All we get is a brief Imagine Spot-like depiction (done by digitally recoloring CycloneJoker Xtreme to have a red left side and Accel's blue headlight "eyes"), since in-story Ryu would never break up Shotaro and Philip's partnership and he'd long since gotten past his hatred.