To be fair, I don't know if you can even say Twilight's popularity centered around its love triangle. Certainly that was a large element in later years, but there was no love triangle in the first book. By the time New Moon came out and Jacob first appeared as a romantic interest, Twilight was already an international bestseller. The love triangle contributed, sure, but Twilight had legs without it.
The Hunger Games was really the first and only of the Big Three that built its emotional arc around a central love triangle. But then again, The Hunger Games also had a much tighter character focus than either Twilight or Harry Potter. Twilight and HP spent considerable time on the romances and personal lives of their supporting casts, whereas HG was very much the Katniss/Peeta/Gale story. They were all crafted a different way and so are apples and oranges - they weren't meant to be compared, and that's where analyzing them for a formula inevitably falls flat. Sure, you can pick out elements that fans responded favorably to in each of those series. But those books weren't successful because of those elements or even because of a particular combination of those elements - people responded to the stories, not a paint by numbers approach.
So really, I think the only thing we can predict about the next big thing is it'll be a story that people didn't see coming - it'll be a huge success because it'll resonate with readers on a personal level that can't be quantified. The only thing it'll likely have in common with HP, Twilight and HG is that it'll be a story readers feel like they've been waiting to read, without even realizing that's what they were wanting.
