It could be a powerful dinosaur itself. In the story, if the dinosaurs have some sort of intelligence or hierarchy, a tyrannical alpha - dinosaur that rules over the others might be the antagonist. It sees the girl as a threat or just as an easy meal and is the one that initiates the act of 'feeding' her to a vore dinosaur.
Maybe a group of hunters from the past. In a scenario where the girl is somehow transported to the dinosaur era, these hunters might see her as a nuisance or a competitor for resources. So, they capture her and feed her to a dinosaur as a way to get rid of her and also to please the 'dino gods' they might believe in, thinking it will bring them good fortune in their hunts.
Well, perhaps in this 'girl fed to dino vore story', the girl is a time - traveler. She goes back to the dinosaur age and is seen as an intruder by a group of dinosaurs. One of the dominant dinosaurs decides to make an example out of her and feeds her to a voracious T - Rex to assert its dominance over the territory.
A powerful alien being with a hunger for anything that moves could be the antagonist. It sees Spider - Man as just another morsel to devour. It has the ability to create portals to swallow its prey whole, and Spider - Man has to figure out how to close these portals and defeat the alien before he becomes a permanent part of its diet.
An old rival of Dino's from his past could show up. This person might have a grudge against Dino and decides to target Hibari as a way to get to Dino. Since Hibari is so proud and strong - willed, he doesn't take kindly to being used as a pawn, and together with Dino, they have to face this old enemy. This antagonist could have some powerful fighting abilities or some dark secrets that make it difficult for Dino and Hibari to defeat him.
Maybe it's an insurance company that doesn't want to cover the cost of the blind woman's treatment. They keep creating obstacles for the doctor and the woman, making it difficult for the treatment to progress smoothly. The insurance company is only thinking about their own profit and not the well - being of the patient.
It could be an evil time - traveling sorcerer. He wants to rewrite history in the prehistoric era so that he can rule the world in the future. He sees the Dino Thunder Rangers as a threat to his plans and tries to stop them from interfering.
It could be. The idea of a girl being vored (presumably swallowed) by a dinosaur is quite terrifying.
There are several possibilities. It might be a powerful industrialist who sees Iron Man as a threat to his business empire and decides to take him down. Or perhaps an alien race that has been observing Earth and decides to target Iron Man first. Another option could be a former ally who has turned against him due to some ideological differences.
It could be a completely new character created just for this story. Maybe some kind of dark sorcerer from an unknown realm who wants to conquer all the other realms.
There's also a possibility that the main antagonist is another student at U.A. High School. This student could be competitive and feel threatened by the girl's relationship with Izuku. They might use underhanded tactics to try and make the girl fail or lose her confidence, adding a layer of drama and conflict to the fanfic.
Well, from the title, it seems the main character would be the gamer girl. But without more details about the story, it's hard to say if there are other important characters. There could be other gamers she interacts with, or perhaps some strange creatures if the 'vore' aspect involves non - human entities.