As seen in these images, each transformed Decade Armor form is a kitbash of the original Rider suit on top of the Decade chest armor that will feature a different text per Rider. The forms Kamen Rider Zi-O Decade Armor will transform into thus far are: Build Sparkling Form, Ex-Aid Double Action Gamer Level XX L and Level XX R, and Ghost Grateful Damashii.
Also, Kamen Rider Zi-O will face his supposed future self, Ohma Zi-O. Why is Ohma Zi-O fighting regular Zi-O?
[Source: Genm Corp FB, SpecialForm12 (link 2)]