A cool thing to imagine would be a world where the land was dominated by megafaunal frog-descendants, as it is/was by mammals today, dinosaurs before the mammals, and non-mammalian synapsids and crocodylomorphs before the dinosaurs.
However, frogs aren't exactly the kind of creature that would be in the position to seize the megafauna niches, compared to other clades like mammals, birds, turtles or squamates. They're not particularly generalistic, poorly suited to terrestrial locomotion and could probably be seriously diminished in the future (due to chytridiomycosis), all that being relative to the aforementioned clades.
So, the playing field would need be leveled to allow them to dominate. However, my question comes here; what natural extinction event could seriously damage the like of mammals, squamates and turtles, but spare frogs? Come to think of it, even within Amphibia salamanders would probably be better equipped to rise.
I'm asking this for a friend, who was wondering. Well, technically, he said he'd be my friend if I figured this out for him, so no pressure...