It may not be possible to merge the giant Monsonian robot--which I have yet to name because of, well, laziness--because it has no soul. It is a machine of war, and nothing more. There is no sentience to speak of when it comes to the robot, and thus it may be exceedingly difficult if not impossible for it to merge with lifeforms that are sentient. Not to mention that the Monsonian government would be furious to see trillions in under-the-table spending be wasted through a powerful weapon's amalgamation into a giant sentient robot.
That being said, giving Iblion a second form would be interesting, especially if it involved the roles being reversed. Basically, Iblion is forced into a new kind of existence where he will possibly regain his powers if given enough hardship and time. alternatively, something different entirely could be concocted.
To better understand my idea/proposal, I HIGHLY recommend that you watch Gurren Lagann. Also, the merger would only be temporary. After Iblion is defeated the robot and CEVA would go back to being separate.
Building onto that; what if the Monsonians find out they can't destroy Iblion, but instead figure out a way to render him into a blank slate. I.e. something to be retaught how to use his powers, as well as being taught morality. It would be awkward for Spencer though, as well as several other parties he's terrorised. I mean, how do you suddenly become friends with an entity that simply forgot it caused so much pain and suffering.
But, before that happens, CEVA and Spencer have to be dealt with, either to win them over to co-operate, or...pacified. Likewise, the COURIER Civil War has to come to a close before they either help or hinder these efforts.