Wouldn't be to sure on any fossil being based on an animal from Hawaii really. No fossil pokemon has ever been from the region the game was based on.
True, just mentioned as it seems like just from the Pokemon revealed so far their trying to stick with animals from Hawaii, Australia, New Zealand, Galapagos Islands, and other isolated islands (in terms of wildlife).
I think they will theme them on tropical animals, but since Hawaii is rather lacking in the fossil department, I am dam sure they will be picking from other areas fossils. Especially as there are some amazing and wacky creatures still untouched; Stegosaurus, Deinocheirus, Therizinosaurus, Platybelodon, Triceratops, Parasaurolophus or Dunkelosteus (looks like a fish combined with a bear-trap, it is horrific).
The reason I suggested Mosasaurus/Megalodon/Plesiosaurus and another Pterasaur was due to me thinking of what prehistoric animal would develop on or around a big island. A marine and flying animal seemed like obvious choices as they wouldn't be land locked so could travel between the islands and the mainland easily. Though GF could break the mold once more and have a fossil Pokemon for each island showing how that island was like in ancient times (though the Hawaiian islands are only a few million years old, long past the age of the dinosaurs).
It just doesn't seem like the right location for a Stegosaurus, Triceratops, or other larger prehistoric animals. However smaller ones are a possible option.
Also, Mosasaurus and Plesiosaurus, not dinosaurs. And as Lapras is a Plesiosaurus as you mentioned, I do not think it will be done again. Also Megalodon is reaaaally boring, it is just a big great white, nothing more.
Details, details. :P Also Larvitar/Tyranitar were essentially t-rex but they made the Tyrunt family. Also Lapras is a very loose Plesiosaur, being it has a shell and horn an much more stouter. And just cause Megalodon is just a really big shark doesn't mean GF has to stay on model, heck maybe they could combine the Mosasaurus and Megalodon together.
One interesting thing they could do would be Alolan form fossils of older pokemon. Rock/Dragon Aerodactyl anyone? Or perhaps Rock/Poison Omastar?
Nah, the reason Alolan Pokemon are the way they are is because they've adapted over the years to Alolan's environmnet. Fossil Pokemon are fossils because they couldn't adapt or evolved into something else.
started from gen 4, and gen 5 only had a single dinosaur fossil.
And don't give me the "pterosaurs aren't dinosaurs", to me and many others dinosaur is already a pretty vague term used to describe the prehistoric reptilian creatures. Pterosaurs, Mesosaur, and Plesiosaur fit pretty nicely in that vague description.
Fossil pokemon actually have very strong parameters, they have to be based on an extinct animal, no more no less. And from a palaeontologist point of view each fossil pokemon is very faithful to the real animals, hell they even went as far as giving the T-Rex feathers once a close cousin of T-Rex was dug up with thick feathers on it.
I'm pretty sure there weren't any bipedal Eurypterids and Anomalocaris.
Not sure how a Rock/Ghost fossil would work, dam thing WAS dead but isn't now :P.
Hopefully this is not a gen where they decide to buck to many trends and kick the fossils out...
I had an idea for a Ghost Fossil Pokemon: it looked like a stereotpyical bedsheet ghost (not unlike the original Lavender Tower Ghosts sprite) with scythe for tar coming from any openings they had. They were the spirits of prehistoric animals that died in tar pits and their spirit couldn't escape as it was trapped inside the body.
Gen II didn't have Fossils so there is a precedence, just like how not all gens have new Eeveelutions.