Well to be honest, I made it waaaaaaaay before everything unfolded with Doflamingo, and how it all played out. But the premise came from the fact that back during Marineford (I think) Law made a statement that they would be enemies one day. And I threw in some stuff about how Law wants to be Pirate King. (Also stated in the same scene maybe? Something along the lines that Raftel wasn't going anywhere) Another one of my arguments was that Luffy has never suffered a true betrayal, and it would be a first, and if it did happen, it should come from someone who was real close to him.
I think the thread did pick up more traction after everything that happened with Doflamingo, and people asked me if I still believed it, because Law is now indebted to Luffy for taking down the man who killed Corzaon, and for awhile I actually argued that they were even because Law saved Luffy's life.
That was the gist of it. I don't subscribe to it as much now as much as I use to, but it still nags me in the back of the head every once and awhile.