Well, here are my ideas:
Ichigo and Nigo - Transform into robotic grasshoppers with high speed, weapons etc.
V3 - This would have 2 effects. V3 himself turns into a large cyclone, which is part of one of the "hidden techniques" that V3 was known for. The cyclone is then used as part of the final attack where Decade joins in. "Cyclone Kick" or something like that
Riderman - He gets a new, more powerful arm-attachment
X - He is transformed into "perfector armor" which is attached to Decade
Amazon - Shrinks into a smaller size and becomes the gigi armlet, which is attached to Decade.
Stronger - Activates his "Charge Up"
Sky-Rider - Turns into a locust based air-glider for Decade
Super-1 - Like Riderman. He gains a new pair of powerful gloves which are used for the final attack
ZX – No idea