WMG / Meg

At some point in a future novel or adaptation, Spinosaurus will appear

The reasoning is pretty straightforward. At first, the series started out focusing only on prehistoric sea life (albeit heavily exaggerated), including the titular shark. However, more recent volumes have included the giant snake Titanoboa and the giant alligator Purussaurus, which were both amphibious freshwater animals, as well as the original Loch Ness Monster, which is portrayed as a giant freshwater eel. The implication, then, is that any aquatic prehistoric animal is fair game for inclusion in the Meg universe. Which brings us to Spinosaurus. It's the largest predatory dinosaur, and (if discoveries made in 2020 are to be believed) the most aquatic as well. Despite all the prehistoric animals that have been featured, the only actual dinosaurs in the series so far was the T. rex and two hadrosaurs in the prologue of the first book. If Alten were ever to decide to introduce dinosaurs to the Meg-verse, the water-dwelling Spinosaurus would be the obvious choice.