In the upcoming All Rider: Let’s Go Kamen Rider! movie, OOO will receive a new combo called TaMaShii. TaMaShii combo is accessed through the Hawk, Imagin and Shocker Medal. The Imagin Medal is created through Den-O body and the Shocker Medal was obviously created by Shocker. OOO will use this combo in order to fight against Shocker.
In the movie, Eiji and Ankh will follow Kotaro and the Imagins back to 1971 with the Denliner in order to fight the Mole Imagin.
Eiji falls on Ankh and Ankh accidentally drops one Cell Medal which they don’t notice and it’s left in the past. A Shocker grunt picks up the Medal and Shocker develops their own Medals from that Cell Medal. This causes the Double Riders to lose and no more Riders appeared after that. Eiji and Ankh return to their own time only to see that the world is controlled by Shocker.
OOO isn’t the only who gets something new in this movie. Momotaros will transform into a sword called MomotaKen (MomotaSword) which NEW Den-O will use.

Before TaMaShii was confirmed, a poster on 2ch claimed that he works for Toei and wrote
“Episode 28 is the 1000th episode of all the Kamen Rider series and it’ll be a special episode. – This has now been confirmed to be true.
Date has an incurable disease, he’ll die and Gotou will become Birth.
The left hand of Ankh will appear.
Kougami has been pulling strings from day 1.
In All Rider, there will be a new combo. Hawk and 2 other, the combo name is TaMaShii. – This is now confirmed as well”
2 things are now confirmed to be true, meaning that the rest is very likely to be true as well.

Scans courtesy of  hZTSdVTdQ and Sanshiki.