I would guess it will pick up where Ep 3 left off.
I think there were a few fan books where Luke restarts the Academy and has a son who becomes a Jedi and some other stuff.
Or they’ll do a pre-prequel...
Star Wars Episode 7 The Musical
It'd be a perfect tie in to where the Return of the Jedi leaves off.
I think that was the original intention, to have VII, VIII, and IX be “the future” relative to III; and IV, V, and VI be “the past” relative to I.
That's the way Lucas originally planned it. Three, three episode series.
Ep 4, 5 and 6 were "A New Hope".
Ep 1, 2 and 3 were the early days of the Empire.
Ep 7, 8 and 9 were supposed to be the further adventures of Luke and his friends.
I have to assume that Disney has purchased all the rights from Lucas to do this. Personally, I have been waiting for this type of news for 20 years.