Board Thread:Questions and Answers/@comment-6527064-20141010110746/@comment-9595671-20141028054305

While they could create the fusions from the original series we likely won't see them. We know from the original Series that the fusions are similar to Kevin's mutations as if certain powers don't work well or oppose each other it limits the fusion's powers like how Heat Jaws powers didn't match at all.

The current fusions we've seen in Omniverse were either aliens with similar powers or aliens whose powers didn't oppose or support the others. TheWalkingTodFernando's list is a result of thinking this as  Fourmungousaur was an example of an alien fusion that has powers that support each other while Atomic X was an example of an alien who's powers didn't support or oppose each other so he had access to both.

So fusions like XLR8 and Fastrack are more likely as if Ben fused XLR8 with Clockwork who's slow then that speed would be cut as a result like with DiamondHead and Big Chuck who are an average of the two aliens used to make the fusion.

While certain fusions aren't completely debilitating they can limit one of the aliens' powers due to the other. Sampy6542's example of Feedback and Amphibian works as it would be Feedback being able to go underwater, so having aliens with similar powers are more likely to appear as they are less likely to hinder the fusion.