HBO has just confirmed that a Game of Thrones spin-off would air at least a year after the end of the final season of GOT.
HBO wants to focus on the last season of GOT before starting anything else, and they don’t want attention to be drawn from the final season.
There are four ideas currently being negotiated/worked on at HBO, but that doesn’t mean we’ll see four spin-off shows, just that there are 4 concepts being considered (it’s unknown how many are actually going to be produced).
The spinoff(s) being worked on do not currently involve any current characters from Game of Thrones, as they will most likely be prequels.
George RR Martin himself has confirmed that the prequels will not revolve around Robert’s Rebellion or his “Dunk & Egg” novellas (which is set in the GOT/ASOIAF universe).
I was really hoping for a Robert’s Rebellion tv show, but since we’re most likely not going to get it I really hope we get to see a show about Old Valyria & the Targaryens…