(Sorry for the iffy pics, but I'm always doing little mode like this and I never post them, so I just jammed this together.)
I decided I wanted to bulk up WFC Optimus Prime, so I took the rocket thrusters from the giant Deadpool double-missile-launcher thing, the one that he stands in the middle of, and adapted them to both WFCOP's alt and robot modes.
Here's the alt mode.


All I had to do was shave the mounting tabs off the add-ons. I removed the long pin holding the folding blue panel on Prime's roof, and replaced it with two shorter pins that stick out the sides a little. (You can see them in the robot mode back shot.)
Then I drilled a hole in each add-on to press them onto those pins, and that's it. The thrusters look great from behind, and all those holes in the front of the add-ons can be guns, launchers, etc.
Here's a robot mode:



I'm using the long curved shoulder panel as the mount, just cut a groove into the underside of the add-ons to fit the tab at the tip. This lets you adjust their positions for posing. He can also rotate his shoulder to use the thrusters for short flights; those little flaps on the back of the arms can be posed for this as well, an nice unintended plus:

FLIGHT!

I really like how this looks. I wish the reds matched tho. Your thoughts please.