Obviously Sam's rant in ROTF served as a trapdoor writing for Sentinel Prime. Same can be said of the fragment of the Allspark. One trapdoor I see that goes all the way back to TF1 and later ROTF is that Megatron almost absorbed every last piece of the Allspark. Another would be if Scalpel managed to keep hidden that shard of the Allspark. Another would be Jetfire's torn spark. Any else you can think off?