Avatar also arrived shortly into the 3D craze and, I believe, was the first film actually shot in 3D (with a separate camera capturing what each eye sees), rather than a 2D film adapted to 3D. 3D hadn't yet taken off and Avatar was the "killer app." People went to see the 3D version, and that was $3 more for every ticket that went to the total gross. They then told their friends and family that they had to see it in 3D and it was just a snowball effect. I doubt that many people actually went to see it in the theaters more than once because they liked the film so much, but because they wanted the 3D experience again (and to share it with their friends and families). Coming out right as 3D was about to explode and being the catalyst for that explosion contributed a lot to the perfect storm that you mentioned, IMO.
I kind of doubt that Avatar 2 touches the record, since, for one, it's not going to have the same fresh 3D experience of the original. Also, few people actually love the original's story, so I doubt that it'll be like how audiences flocked to The Empires Strikes Back because Star Wars was universally adored. Finally, since there are going to be four sequels, some people may just sit the first one out, since they'll have three more chances to see something similar in the theater and may even want to watch them all back-to-back. I'm sure that it's going to be a blockbuster smash that likely breaks the $1B barrier, regardless. I'm just skeptical of it breaking the record, though I could be wrong.