This is a question that a lot of people have wondered about and were ready to give voice to when Transformers: The Last Knight came out, but have kind of cooled to ever since. There are still plenty of people that want to see Unicron make an appearance on the big screen, but given that he appears to be buried in the earth it’s bound to be kind of tough unless a reboot can turn things around and wipe away everything that’s come before. It’s been done before after all, but with Unicron, it might be absolutely necessary since having him bust out of the earth and tear things to pieces on a global scale might have to wait until humans learn how to leave the planet or have no choice but to do so. The fact is that Unicron is HUGE, and not just gargantuan, D&D elder dragon huge, the character is a PLANET unto himself, as anyone could learn by doing a little research on him. The only problem with representing this on the big screen would be figuring out how to make it feasible when it comes to the story.
Think about it for a second and then it should hit you, Unicron is the size of the earth or bigger. He might even be the size of Jupiter or bigger since he can literally eat worlds. In the last Transformers movie it was noted that he was buried in the earth, or rather, WAS the earth, which is even more confusing since it would mean that somehow, someway, Unicron became a terrestrial planet, or was somehow surrounded by one. The truth is that Unicron is one of the two gods that helped to create the Transformers as the other one, Primus, is the younger sibling that was split from Unicron’s body apparently and ended up giving rise to the Transformers when he sacrificed himself to become Cybertron.
That would actually offer another explanation as to why the planet-sized robot attacked Cybertron in the 1986 animated movie, but it’s fair to say that a lot of fans didn’t know this since the canon probably hadn’t developed quite that far yet. If it had, then it was still largely unknown since like it or not, people enjoyed the Transformers primarily because they were robots that could turn into vehicles and fought against each other for one reason or another, creating an interesting story that was extremely popular for a while. When Unicron came into the picture though it’s easy, to be honest and state that he was something of a surprise, not just because he was voiced by Orson Welles, but also because he literally consumed planets, along with anyone that happened to be inhabiting them at the time. He didn’t really care who he consumed, but it was shown that Unicron had a special hatred for the Transformers, and not just the Autobots either, since he gladly attacked Decepticons as well. That might make it feel a bit odd that he would spare a wounded Megatron and his minions, but it made sense considering that Unicron is the type of character that utilizes minions to do the work that he’s simply too big for.
There was only one thing that kept him at bay and that could hurt him, and that was the matrix of leadership, which Optimus Prime, who died in the animated movie, passed to Ultra Magnus, who was blown apart by Galvatron in turn. When Hot Rod defeated Galvatron he then used the matrix to defeat Unicron, thereby becoming Rodimus Prime, a new leader for the Autobots. Sadly, the Hot Rod that we’ve seen on the big screen is kind of a goofball that has a time-stopping gun and an extremely flippant attitude, which is kind of like the animated version but still very different. Plus, we haven’t seen Ultra Magnus, Cyberton has been ripped apart by a black hole, and once again, Unicron is buried in the earth somehow, so bringing him out to play would be kind of tough. This is where the animated version got it right at least, since leaving Unicron in space to wander about, spreading chaos and havoc along the way, was a much better idea.
If we do get to see this character in the movies it would be something of a miracle since the upcoming Transformers movie that has already been talked about is focusing more on bringing different versions of the Transformers together as Beast Wars is getting a push and it would appear that Gen 1 Transformers are being used as well. At some point, it would be great to see if someone is going to be ambitious enough to bring Unicron into the fray, but until that happens it’s likely that the Autobots and Decepticons are going to have plenty to fight about between them.