Guys, I have been thinking about somethings that it could calm my nerves about my previous theory, about the fact of Ultimate Humungousaur's helmet appearing in his ship as one of Khyber's throphies.
As we have been told, the Nemetrix, the device used by Khyber's pet to transform itself into different alien predators, was created by an evil genius Cerebrocrustacean named Dr. Psychobos. This character still has to debut in the series, but he have already seen an official artwork of him. It's funny, you know? Because being an Omnitrix-type device, I suspected its creator could be Albedo. According to information revealed by Derrick J. Wyatt, Dr. Psychobos considers himself and Azmuth to be rivals, but Azmuth doesn't care. He also said that Azmuth is not aware of the existence of the Nemetrix; and also that Khyber has had the Nemetrix for over five years. (Only one year less that Ben has had the Omnitrix). Well, considering this rivality, I think it's obvious that Psychobos have been spying Azmuth and he has possibly stolen some of his invention's designs and other technologies. Reason?, because as his "rival", he want to prove that he can create something 10 times better that anything Azmuth can. That's probably the way Psychobos created the Nemetrix, modifying some planes or data that he stole from the Omnitrix's designs.
That makes me think, if Psychobos could stole the Omnitrix's designs to create his own version (the Nemetrix), who says he didn't do the same with the Ultimatrix? I'm pretty sure that this Dr Psychobos would find the Ultimatrix's evolutionary function very "fastinating", even although it was actually Albedo who created it instead of Azmuth. Anyway, if Psychobos could also steal and copy the evolutionary function of the Ultimatrix, maybe he has a device able to do the same thing. He could have first tested his device with a Vaxasaurian, successfully turning him into a copy of "Ultimate Humungousaur". Then Khyber, fascinated with the new creature, decided to hunt it and turn it into one of his throphies. That would be the reason because Ultimate Humungousaur's helmet was in Khyber's ship. I mean, you said Derrick confirmed that was from "an" Evolved Vaxasaurian; but what's the point of saying "an" when there's only one individual as far as we know? Unless, of course, that means there are actually more that one individual; and we simply didn't know yet.
You can think my theory is improbable, but the truth is that is not impossible. Think about it. We don't know what exactly happened with the Sentinel Ultimate Forms after Azmuth took them with him. However, I have a theory that Azmuth took them to live in peace on Primus, where they will be its protectors to defend the planet for any possible invaders, like Vilgax did in "Alien Force". In anyway, the point is that Azmuth would keep an eye on the Ultimates wherever they were, so he would know if they go rogue or something. In "Troble Helix", we saw how Khyber like to be in the shadows, meaning that he doesn't want to draw attention to himself. If he had attached and killed Sentinel Ultimate Humungousaur, I guess Azmuth would have discovered him; therefore he wouldn't be able to act as he pleased anymore. Besides, apart form the Sentinel Ultimate Humungousaur, I think someone like Khyber could find another "worthy prey/trophy" in the Sentinel Ultimate Spidermonkey; and I haven't seen anything in Khyber's ship that could seem a part of Ultimate Spidermonkey's body. Finally, I guess that the other Sentinel Ultimate Forms would have try to avenge their leader (Ultimate Humungousaur) in case Khyber had killed him.
Well, what do you think? I'm pretty sure my theory is right, which still allows the possibility for the return of the Sentinel Ultimate Forms.