Jesus Christ with the Marines already.
Why should they just stick with what you want? The Dimetrodon is one of the most reconizable prehistoric reptiles, so recognizable in fact the toys are almost always sold with dinosaurs, the same was pterosaurs. I'd love to see them, a long with other Quadrepedal Carnivores that would add a nice dynamic too the list of enclosures in the park...that isn't just a giant lagoon of marine reptiles that are seperate and unequal the the rest of the dinosaurs in your park.
I get the appeal..Mososaurus, and even more important in my opinion, a pleasiosaur, would be cool to see, but don't try and play favorites against something as recognizable as a Dimetrodon. Please and thank you.
Personally, I don't really care much if they do or don't add pterosaurs or marine reptiles. But, I understand that a lot of fans want them in the game because of their connections to the franchise, especially animals like Pteranodon and Cearadactylus which are canon species. Personally, I think they need to increase the diversity of dinosaur types before they start adding non-dinosaur species.
Also, just because it's recognisable doesn't necessarily mean it would fit in the game. I like Dimetrodon, it's an interesting species, and I like it just as much as any other prehistoric species. However, the Jurassic Park/World has focused on species, mainly dinosaurs, from the Mesozoic era, whilst Dimetrodon existed before the Mesozoic, so I don't think that it would fit (again, this is just my opinion).
As you said, the developers are the ones that will decide what they want in the game. They may want to add Dimetrodon and other Permian species to the game. And if that's what they decide, fair enough.