I don't read the Gaim novel, but the Japanese fans have confirm that there are new Arms Change in the novel. These are Zangetsu Suika Arms, Zangetsu Jinba Melon Arms, Ryugen Jinba Dragonfruit Arms.
The main enemy is Saver and he have a lot of transformation, all based on big bad of Gaim such as Rengesa Gishin Saver (Lotus Seat Fake God Saver, fuse with Bujin Gaim's tree), Saver Golden Arms (Kuro Zakuro + Golden, not Kougane's Golden, created from Saver attempt at artificially creating the Golden Fruit), Saver Darkness Arms (Kuro Zakuro + Darkness), Majya (The big bad of Gaim Final Stage), and Skeleton Dinosaur (Great Leader of Badan).
Decade/Tsukasa apparently appear as a cameo helping Mitsuzane when he was attacked mentally by Saver and the other Heisei Riders from Kuuga to Drive and their enemies (funnily, the enemies of Ryuki's world are Kamen Riders instead of Mirror Monsters) are referenced, and the novel pretty much use all of Gaim related movies as materials for the novel. There's also a chronological timeline of Gaim in the last part of the book