With the knowledge that Prime killed Demolisher at the start of the movie and that sometime during the movie Bumblebee killed Rampage, I had this theory:
Over the course of the movie the individual Constructicons would be defeated by the Autobots. At the end of the movie the fallen Constructicons would be rebuilt to combine into a giant robot so they could have their revenge. I also liked the idea that the central robot was renamed Scavenger as the Decepticons were using scavenged remains to build their ultimate weapon.
Turns out the Ultimate weapon was buried in a pyramid, The Fallen was the name of a Decepticon and Rampage was killed after Devastator was formed... but it was a good theory otherwise