So Jurassic Park in a world with wizards. Taking fossils and bringing them back to life. Like undead? Could be. Dinos? Maybe. Like a vampire raptor would be horrifying. Archaeologists are scraping away at this skeleton when one of them scratches his hand. A solitary drop of blood rolls down his finger. It reaches the ancient skull...clever girl.
(lifting the script of the movie as a whole and simply replacing dinosaurs with vampires would make a rewarding and possibly hilarious adventure. "i have spared no expense" for nice coffins and finest virgins' blood. Instead of herds of herbivores the PCs see some sophisticated salons where tired gothic lesbians recline and complain of ennui. Wayne Knight ruins the system of holy symbols and running water, letting frustrated immortal teens looking for love mob the players' transport. Also suggesting the bard player do goldbloom impressions the entire time would be worth it by itself)
I don't exactly want to do straight dinos though. The bit about using frog DNA makes me think of slaadi, and the fact that they infect humanoids to progenate makes it perfect. The toadlike ancients are perfectly preserved, but some weird colored dust was kicked up and now a nasty flu is going around the diggers' camp and the priest has been looking a little green.
Instead of living in Limbo or wherever cosmology you have they were just ancient rulers of the dinosaurs. An amphibian/fungus hybrid race. If you don't have myconids give these guys all their abilities too, like confusion spores. Miners break into a slaad grave and start going crazy and burping up clouds of spores on each others' faces. wait that's a little too 28 days later.
Okay miners dig up a weird froggy skeleton and start getting sick and growing fungal lumps and their features start looking decidedly toadlike. This is all covered up by the mining company and quarantined. A wizard who loves-knowledge-and-doesn't-count-the-ethical-cost decides to "breed" some reds in a big petri dish, like an island or life-sustaining cavern. Keeps the human host thing secret and engineers it somehow so they don't produce more spores. Gets aristocrats from the nearest decadent civilization to invest in the curiosity of the ancient masters of the earth. Somebody gets attacked by a crazy frogperson and shows signs of transformation. Wizard hires PCs to distract/protect the rich folks while he finds out why. Act 2 a new big blue slaad appears and attacks the rich group. Act 3 a semi-infected green has been sabotaging security. Wizard shows that maybe he is the real monster here etc etc.
I like all this stuff but it isn't sandbox material so probably just stick with "miners dig up something that is making them crazy" or like "people drinking from a certain stream look froggy" and have it be one of several factors in a dungeon or town instead of THE thing