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.
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.
Kirihiko's little sister Yukie/Yuko in "Y's Tragedy" is Kaoru Hirata, who played Yuka Yamazaki, also in Mahou Sentai Magiranger.
The human form of Arms Dopant? That's AbaRed (Kouichirou Nishi). As if to push the point further, he states repeatedly that he loves to "rampage"note "Abare". And on top of that, Shotaro and Philip fight him with what is basically the Double equivalent of Abarekiller, spines and all.
And HurricaneBlue (Nao Nagasawa) appears in "The D was Seen" as Lily Shirogane, a stage magician who uses a Dopant Memory to cheat at her disappearing act. She would return in the Accel edition of Kamen Rider Double Returns as a waitress at her grandfather's coffee shop. She is still a magician, though, and even manages to allow Shotaro and Philip to escape police custody via her Memory-free vanishing act so they can assist Accel.
Amane Kurihara from Kamen Rider Blade (Hikari Kajiwara) shows up in "The Vengeful V" as one of the art students.
The actor playing the human form of the Liar Dopant was in Kamen Rider Kabuto, playing a surgeon that rotated his duties with two Worms mimicking him.
Kirihiko showed up way back in the first two episode of Den-O as one of the bullies who harassed Ryotaro.
Of all the Japanese comedians who get bit parts in this show, the human form of Cockroach Dopant is a member of the comedy group Rahmens, best known for his role as "Persocom" from the Japanese Mac and PC commercials.
The teacher from "Find the C" which accosts the Dance Battler Dango could very well help Double with his crime-solving exploits as he is, after all, a retired Metal Hero.
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, Genre Savvy that he is, didn't freak out at the fact that his partner's true name is RaitoSonozaki.
What Could Have Been: "The O Chain" storyline has Shroud mention CycloneAccel Xtreme, and we even get an Imagine Spot-style image of it, but it doesn't happen and probably never will because it would involve breaking up Shotaro and Philip's partnership as well as fighting with hatred rather than compassion, two things Ryu refuses to do.
Though we did get CAX in Kamen Rider Climax Heroes OOO.
Word of God: The series itself is confirmed by the staff to be a homage to the Showa Era, from the little things such as the scarf to the overall feel of the series.