Okay.
Imagine, he's just like Ben Tennyson in Kamen Rider Multiverse.
Riders powers:
Kamen Rider Decade can use KamenRide Cards to transforms into other Kamen Riders and gain their powers and stastics, can use FormRide Cards to transforms into another form of other Kamen Riders and of course, gain their stastics.
AttackRide Cards:
He can use AttackRide Cards to use other powers of himself (and other KamenRide), such as Duplication, Invisible, and others.
Power Mimicry:
He can copy other powers through his cards, such as Shinken Red's Rekka Daisanto.
Existence Erasure (Uncontrollable):
Acidentally erase Kiva and Hibiki, along with their universe.
BFR:
Can send his enemies into a dimensional rift.
Time Travel:
With a dimensional wall, he once travel to Muromachi period.
Intelligence:
Should be Gifted or Genius. Calculative, cunning and manipulative, he became Leader of Dai-Shocker since he was young, outsmarted many villains, manipulate every Kamen Riders (Including the one that has an IQ of 600, twice), Super Sentai, Dai-Shocker and Zangyack (Well, an entire film was just all of his plans). Very skilled fighter, quickly infiltrated a parallel world with ingrained information on that reality.
And about his cards, I would put the list into his page too.