This movie contains examples of the following tropes:
All There in the Manual: The names of Michal's Kamen Rider forms. The evil form that uses core medals is called Kamen Rider Poseidon, and the good form created from the water-powered belt is called Kamen Rider Aqua.
It's implied that Foundation X backed the Kougami Foundation's research on Core Medals.
Shotaro still uses the Lost Driver though it's unclear if it's the one left to him by Skull or the one left to him by Philip after his "death"
Both crossover cameos in Kamen Rider W Forever and Kamen Rider OOO Wonderful are referenced.
Not to mention that Shotaro references Eiji's exclamation that "Riders look out for each other" from Kamen Rider W Forever
The Seven Legendary Riders are battling Foundation X in various countries. Back in the old days, when a Rider's predecessors would always appear in the final arc to help out, where did the guest star Riders say they were coming from? Fighting the evil organization's branches in other countries. This goes all the way back to the very first series, when Takeshi Hongo - his actor having recovered from the injury that necessitated Hayato Ichimonji's introduction - returned midseries. Fighting Shocker overseas is where he'd been. The only difference is this time we see the Seven Legendary Riders' overseas battles before they return to Japan to fight alongside the current heroes.
Early-Bird Cameo: As each Movie War movie has featured the introduction of the upcoming Secondary Rider of the current show, Kamen Rider Meteor makes his first appearance at the end of the movie.
Eleventh Hour Ranger: Kamen Rider Aqua returns from the future to help the Riders against Foundation X's forces, and give Eiji the Super TaToBa medals.
For Happiness: Shotaro is quick to voice how unimpressed he is with Gentaro - and just as quick to change his mind when he learns they have the exact same motive.
Ham-to-Ham Combat: Shotaro and Gentaro, with Eiji and Philip looking on in (respectively) bafflement and amusement.
Men Don't Cry: Gentaro claims that a man is only allowed to cry if he's lost his wallet or been dumped.
Mooks: Through their research into the Gaia Memories, O Medals, and Cosmic Energy, as well as their partnerships with the Museum and Gamou, Foundation X boasts a vast foot soldier army of Masquerade Dopants, with a white suited variant transformed from Foundation X members using Gaia Memories, Waste Yummies, and Dustards. Poseidon also creates his own Waste Yummy army in OOO's portion by channeling the power of Uva's Core Medals, while in Fourze's portion Virgo leads a force of Dustards against Fourze and Nadeshiko as a favor from Gamou to Foundation X.
Sealed Good in a Can: The Seven Legendary Riders are sealed inside Fourze and OOO's transformation trinkets by Kannagi. 1, 2, V3 and Riderman are trapped within Astro Switches while X, Amazon and Stronger trapped inside Core Medals. They are freed when Fourze and OOO perform a Limit Break and a Scanning Charge respectively.
Super Mode: OOO's Super TaToBa Combo and Fourze's Rocket States.