UPDATE:- Borderlands: The Handsome Collection has indeed been confirmed as the latest Epic Games Store giveaway, meaning it's all but certain at this point that ARK: Survival Evolved will be the next free giveaway.
Here's an interesting one - PC Gamer has noted that a recent image posted to Reddit, which claimed to have details of the next four free games to appear on the Epic Games store, may just be more accurate than first thought.
The image showed GTA V, Civilization VI, Borderlands: The Handsome Collection, and Ark: Survival Evolved - at the time, GTA V had already become available, leading many to question if the image was just a hoax. Interestingly, however, the second game on that image, Civilization VI, is indeed the next game Epic is giving away.
It could still be a coincidence, of course, but if it does hold true, it means Borderlands: The Handsome Collection will be next, which includes both Borderlands 2 and The Pre-Sequel, along with all the DLC.