GRRM's preferred title might hint at the plot
Everyone makes mistakes, including George R. R. Martin. But luckily, when he accidentally announced what the maybe/maybe-not-confirmed title is, he inadvertently revealed what events the prequel could chronicle. As GameSpot pointed out, the Age of Heroes was 8,000 years before GoT and disrupted by a painfully brutal winter that ravaged kingdoms -- what is known as The Long Night -- and grew even more destructive when the White Walkers arrived for the first time to Westeros. But eventually, they were defeated once an unknown figure called the Last Hero joined forces with the Children of the Forest to see their demise.
Will the darker period that’s said to be coming in the series be The Long Night? And could Watts' character's secret be that she is the Last Hero? At this point, it's impossible to say, as this is all just speculation of what the Goodman-written show might cover -- but the events do lineup in the vague description of the plot, so at least a mention of this moment could be expected.
What is the release date?
Don't get too excited about the prequel yet, because it might not arrive for a while. Right now, HBO is focused on building excitement for the final season of GoT in advance of its release date in April 2018, and given that the series has yet to start filming, a 2020 debut seems like the earliest possibility. HBO president of programming Casey Bloys, at a Television Critics Association panel in 2017, said, "The number one priority in all of this is the final season of Game of Thrones... I don't want to do anything with a spinoff or anything that detracts or distracts from that."
Fans of Thrones are no strangers to waiting, though, so you can hold out for a bit longer. But in the meantime, check back for more updates as they're announced.