If they're all ancient zoans, we have two options:
First, they're all Ryu Ryu models, which I'd find to be boring as hell, to be honest. But if this is the case then the possibilities aren't too wide since the number of popular saurs is reduced.
But if they are more diverse, maybe we should think of what common zon models we have in order to guess the ancient versions Oda may introduce. Let's remember that, apart from the Ryu Ryu family (which seems to follow Kaido's own fruit, that very likely is the mythical model), every abnormal zoan is the version of a common family: Zou Zou, Inu Inu, Tori Tori, Hito Hito... We've seen them all.
Given this, we could potentially see these ancient models:
Tori Tori; terror birds should be the more expected animal. Dodo would be funny, though, and archaeotperyx a bit underwhelming compared to other birds and non-avian dinosaurs. The rest of candidates seem too unknown (argentavis, Haast eagle, etc.) to appear in a mainstream manga.
Hebi Hebi: Titanoboa is probably the best candidate, but I don't really know how Oda could differentiate it enough from big users of common snake fruits like the Boa sisters.
Inu Inu: similar to Titanoboa's case, not known nor distinguished enough to deserve a fruit, in my opinion.
Mushi Mushi: now, this is more likely. If Oda puts every arthropod under this category I'd certainly see an anomalocaris fruit. You may be thinking "man, nobody knows that thing", but in Japan they're absurdly popular and you can find them in lots of anime and manga. Other creatures like the arthropleura, especially, or the meganeura are also contenders.
Uma Uma: considering that Oda understood Kaku's fruit as a model of the Ushi Ushi (cows), I assume he could do something similar with perissodactyls; therefore, the rhinoceros fruit would be an Uma Uma model. The rhinoceros fruit exists anyways (one of the beasts from Impel Down), and the wooly rhino is the most attractive option here, I think.
Neko Neko: well, this is obvious. Sabertooth.
Sara Sara: a beelzebufo fruit should be expected if Oda makes an ancient model of this power. A big and horned frog would be interesting and relatively easy to differentiate in terms of its visual, especially if he stresses the whole "devil toad" etymology. In fact, this was one of my bets for Queen's fruit along with the brachiosaurus he ended up having.
Kame Kame: while the most known prehistoric turtle is the archelon, and we've seen in other Japanese media, I find it to be too related to water so I don't know if Oda would go that route. Also, one of my bets for Ryokugyu's fruit is the mythical model of this fruit (that or the Toki Toki/a time paramecia), and I don't think we will see the ancient version.
Mogu Mogu: I love moles but they aren't much versatile. If Oda goes crazy he may put armadillos and therefore glyptodonts under this category based on some ground-related reasoning, but I'd find it to be a stretch.
So my bets: either they're all dinosaurs (ankylosaurus, tyrannosaurus, parasaurolophus... creatures well-known) or they are, let's say, a sabertooth, a wooly rhino, an anomalocaris and a terror bird (Neko Neko, Uma Uma, Mushi Mushi and Tori Tori). If I give them to each Roppo:
Sasaki - Sabertooth.
Black Maria - Anomalocaris.
Who's Who - Terror bird.
Ulti - Wooly rhino.
For example.