Fang Joker (as pictured above) is the next Double Form. It is activated via the "Fang Memory", which exists in a Raptor-Like Memory Gadget. The Fang Memory can clip onto the Double Driver, and it is assumed that it can give Double the forms of (Fang Joker, Fang Metal and Fang Trigger.)
Recent rumors divulge that Fang is given to Shotaro as a replacement for Philip, who seems to die in episode 25. I don't believe Philip will stay dead of course, but it's a great way to get Double a new form without it feeling forced (since he will loose the ability to use cyclone, heat and luna.)
Other rumored big events include:
-The Second Rider showing up. His name is Ukyou Suji (last name has the character for "right" in it.) He will be a Genius Physicist who works with the police, and my guess is he has some link to Philip. His Gaia Memory appears to be "Accel".
-Ukyou's partner, Mira will also appear. She is a girl who seems to have a similar past with Philip, and her story ties into the "Begins Night."
-Kirihiko will overthrow Sonozaki and control a new empire in his own name.