Yeah they revealed Tyrunt and Amaura before XY's release, I remember as the internet went crazy we're finally getting a T-rex Pokemon.
If they decide to strictly stick to extinct animals of Hawaii I guess the duck and maybe a combination of a snail and coral reef would be interesting. However now would be a REALLY good time to do one of the big ocean fossils like a mesosaur (the marine dinosaur you saw in Jurassic World) and/or megaloden (the big shark)(I guess also the plesiosaur if they want a more dinosaur version of Lapras). If they don't want two marine fossils I wouldn't mind seeing another pterosaur, especially ones with the odder beaks/crests.
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.
Kabutops, Omastar, Carracosta, Cradily, Armaldo are all aquatic so have no ties to any landmass.
Aerodactyl is a Pterodactylus, normally from Europe and Africa, but placed in Japan(Kanto).
Tyrantrum is a T-Rex, so from exclusively North America, but was in the region based on France(Kalos).
Auroras, based on Amargasaurus from South America, also put in France(Kalos).
Rampardos is based on Pachycepholosaurus from North America, put in Japan(Sinnoh).
Bastiodon is a Chasmosaurus from North America, also in Japan(Sinnoh).
Archeops is an Archeopteryx, from Germany, placed in New York(Unova).
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).
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.
One interesting thing they could do would be Alolan form fossils of older pokemon. Rock/Dragon Aerodactyl anyone? Or perhaps Rock/Poison Omastar?