So remember the truck and often times the little girl or little boy that are involved in the Isekai's transportation or reincarnation, I want to say that every single isekai Kai that has the truck takes place in the same universe. And the truck and the little girl or little boy that the main character saves is one entity and his purpose is to search out people who are heroic enough to die for the small child in order to transport them to set world to save it.

Feel free to mess with this theory as much as you want I was just watching isekai kind of suddenly hit my brain

As an added to this theory, truck-kun, it's an Eldritch horror type creature. . . That gains power from throwing people into other worlds, eventually he becomes so powerful that he overthrows the original world that all the heroes came from and they have to run from respective worlds back to their original World in order to save it they all team up in this isekai star team and fight truck-kun in the ultimate final battle, I love to see people like kazuma and Subaru and nafumi, and many other popular isekai protagonists showing up in a epic battle. I wouldn't even mind if it was just a movie as long as it was really well animated though it have to be like a three to four hour movie with the first half being all the different isekai protagonist getting to meet each other and having conversations and s*** and talking about the respective fantasy worlds and experiences, before discovering what exactly the source of the autonomous power that brought them here is. Before finally teaming up and taking it down. It also be cool to have like the favorite side member of each group, it probably end up being Rem, Megumin, and Raphtalia