I want to have some bad guys from previous films return and team up against the Gang of Seven. My current idea is to have Sierra, along with Rinkus, encounter Dil and Ichy in the Land of Mists, all realize that they all dislike the Gang of Five, and have Sierra suggest an alliance and actually try and make good on his "I'm going to feed those overgrown eggs to the Sharpteeth!" threat. Also, it would be fun to see Three Talking Sharpteeth (Chomper, Dil, and Ichy) meet finally and see what they'd say or if they'd go

when they realized that they weren't the only Sharpteeth that could talk.
Also, for the fun of it, in that one, I'd bring in Pterano and Guido too to help the good guys.
And have Chomper and Ichy in a fistfight at one point.