That'd be pretty awesome, but would these small theropods be fundamentally different from the large predatory terrestrial birds of OTL? (considering they're in fact quite closely related)
Well, we could simply take out the terror birds and replace them, one-for-one, with troodontids, and allow evolution in South America to the follow the same course as in OTL, with glyptodonts, giant sloths and toxodonts. But, since the OTL terror birds went extinct before humans ever came to the new world, we'd have to assume that the same fate would befall the troodontids if they were essentially interchangeable. This obviously doesn't accomplish what we wanted with this timeline, so we probably shouldn't follow this route.
Also, troodontids probably hunted differently from terror birds: they had arms that would be used in hunting, and their feet seemed quite robustly beweaponed as well. If we follow the heuristic that form follows function, this should suggest a claw-based prey-capture behavior. By the same heuristic, we would conclude that terror birds probably used their massive, axe-like beaks in some way.
So, given the different anatomical arsenals, I would think it a stretch to simply replace terror birds with troodontids. Undoubtedly, the evolution of things in South America would be quite different. Also, these troodontids would have first appeared and become dominant predators early in the Paleocene (65-55 million years ago), whereas terror birds didn't really become big until the Miocene (23-5 Mya): thus, they would have diversified and started strongly impacting the evolution of other animals earlier. We might see a huge diversity of them, ranging in size from chickens to ostriches, with a whole suite of unique behaviors and lifestyles.
