If Gen 3 is anything to go off of (based on how Hoenn and Alola are both tropical regions), I'm not going to expect to see a Fossil Pokemon based on a dinosaur this time around, but hopefully I'm proven wrong. Gen 3 gave us Lileep and Anorith, which are both based on creatures from the Paleozoic era; with Lileep being based on Crinoids and Anorith being based on the shrimp-like Anomalocaris. With that in mind, I would love to see a Fossil mon based on this guy:
It's an extinct shark called the Helicoprion, which died out during the Triassic period. I know we already have sharks, but can you imagine a shark-like Pokemon that can extend its mouth like a whip to attack the opponent? Not only do I think that would be cool, but the mouth thing could also act as a unique weapon.
I also want to see Fossil Pokemon based on extinct mammal species. If memory serves me right, I don't think we've seen that implemented yet. I really would like to see a Grass / Rock type based on this guy:
It's called the Entelodont, also known as "The Pig from Hell". It lived during the Eocene epoch and was one of the top predators of its time. It was mostly an omnivore, but their adaptations show a bias toward eating meat. Entelodonts had very powerful jaws, which could crush bone, so in Pokemon-terms, I can see a Pokemon based on this guy having the Strong Jaws ability.