I know, I know... there's been a lot of talk about what the plot is going to be about, or which famous (or not famous) actors should appear, or even if there's going to be any EU charachters in the film... But what about the real subjects of themes that fans or new fans MUST have? Here I present my opinion.
1. The dark side of the Force: Many of you will object that we got to see that during the Original Triology, but all those who have read "Book of Sith" or any book about the times before Darth Bane's Rule of Two (you know, before the Jedi started thinking Sith were extinct), know that there's plenty of material for the development of characters, plotlines and the equivalent to a gold mine in information, theory and movie material, such as amazing new abbilities for villains and completely new (at least for the movies) charachter's philosophy.
2. The new Jedi Order: Yeah, I know, it's obvious that it's going to be there. But I don't just mean the mere appearance of it, but also a profundisation of it's way of life. The Order that we knew was completely wiped out, there's no one left from that time alive to remember their ways. And Luke doesn't think the same way as those old masters, so probably, as explored in the EU, this Order will be much more modern and less strict than the older one. It's very likely, being himself the son of a Jedi, that Luke will allow Jedi to marry each other, and this Order probably won't force it's members into seclusion from the outer world. It would be very interesting to see their development as a society.
3. New kinds of Force users: We don't really know what the plot is going to be like, who are going to be the bad guys and what is everyone going to be fighting for... So maybe, both Jedi and Sith will have to join forces in a world in which a new kind of enemies, a group of people that can use the Force (or even other sorts of power) in a way they never imagined possible, which threatens the existance of all traditional Force users (theoretically, Sith and Jedi use different sides of the same thing, the Force, which makes their abbilities transportable, which means, a Jedi can learn almost without help to use the Dark Side and viceversa), or even, taking it a step too far, their conection to the Force in itself, or even the way the universe works according to their laws...
Well, I have other theories, but they need more work... I hope you beautiful readers found this interesting, thanks for reading!
(if you need further explanation, please contact me or coment below! ;D)