$3 billion worldwide. I think with a great story, great cast, great director, and an opening day that is the same as the highest-grossing film of all-time worldwide (i.e. "Avatar"), it looks like it could make $3 billion worldwide.
Avatar had the advantage of being a new name though, Star Wars brings a lot of package, both positive and negative, as legendary as the original trilogy is, by now, almost everyone has an opinion about it, and i do see many having a negative perception, be it that they didn't grow with the films or were nver able to get into it.
For this film to succeed, i think it's going to need to feel like a real fresh start, something everyone could get into, pretty much how Avengers didn't need everyone to have seen the Phase I movies. Marketing can also pay a key factory, if the trailer is interesting, it can get anyone interested, even if that person doesn't generaly like the brand, the latest Godzilla trailer was a pretty good example of a great trailer, and that film didn't make more due to the film itself being a bit of a disapointment, leading into poor word of mouth.
I think a Billion is guaranteed, even if the film ends up being bad, 2 Billion if it can give a wow factor like Avatar and Titanic did, but i think it's a bit doubtful.
Old fans are already interested in watching it, so i think that the keys to making this film break Avatar's record lies in:
1) Perfect marketing, like the Godzilla example i previously i gave, the kind of marketing that makes even people who have a negative opinion on the franchise get a bit interested in knowing what the fuzz is about.
2) Wow factor, not sure we'll get this though, nowadays this is very difficult to do, the only upcoming films that seem to be doing that are the Avatar sequels due to having James Cameron constantly pushing technology in film. But then again, it's not impossible, even if you don't have new shinnier effects, the Millenium Falcon scene from the teaser was quite dynamic.
3) Rewatch value, after seeing Avatar, Frozen or Guardians of the Galaxy, may wanted to watch those movies again, Episode VII needs to be able to be that kind of movie, something people don't get tired of easily.
4) Conquer a new generation, no offense, but i think the older fans are already a given, but i don't think we are enough to make this film dethrone Avatar, in order for that to happen, you need to make a new generation obsess over Star Wars. Avatar was the kind of film almost everyone was talking about, i think that for teens to people in their early 20s, Facebook is a good place to evaluate that, you can see how popular Harry Potter and both top grossing James Cameron films were. Going from Facebook, even the fast and the furious, and the Transformers films are more popular, and to be honest, from the people in that age demographic i speak with, that notion seems to be correct. The Avatar and Titanic pages show that both those films spoke well with this demographic, so, i think it shows that for a film to break the record, it realy needs to conquer people of all ages.
Taking all these factors into account, i think it's doubtful Episode VII can beat the record set by Avatar, it already has too much package. However, beating Avengers may be possible, it's going to be interesting to see the end results of both Avengers 2 and Episode VII by the end of the year.