Please read this Carefully!
Kamen Rider Build will be using the Full Bottles of Game & Doctor and transform into a Form called Kamen Rider Build Ex-Aid Form which looks exactly like Ex-Aid! The Full Bottles of Game & Doctor were made by Misora from the Ex-Aid Full Bottle taken by Build during True Ending.
The new Rider ver. Kamen Rider Heisei Generations FINAL poster revealed a new type of Guardian. This Guardian had 'X' on their chest & face. By seeing this it reminds me of only one Organization which is still in mystery. That is Foundation X.