Would it be possible that large flightless predatory birds similar to the extinct terror birds, descended from seriemas or birds or prey that have given up flight, could evolve in the future if the large mammal carnivores were to disappear in the region, and the birds themselves were isolated from anywhere else?

This question really is no different from your last one on dinosaurs - sure it's *plausible* that given enough time major changes like shifts from herbivors to carnivory, flight to flightlessness and especially increase in size could occur. It has happened multiple time in multiple lineages (look how many times animals have goen from the water to land and then back again later for example). Working out which might do what in the future however is increidbly hard since it is hard to predict what a specific lineage will do even if we can often sayn on average what is likely to happen or what is possible.