In truth, all we really knew was that the show was going to be very, very expensive. According to reports that have come from SlashFilm and TheOneRing.net, Amazon's series will focus on stories from Middle Earth that haven't yet been adapted into films.
When we meet Aragorn in The Fellowship of the Ring, he's a mysterious figure known as Strider, an heir to the throne of Gondor who is battling the fear of failure for the generations before him. In his youth, Aragorn was unaware of his lineage, and only discovered the truth when he was older. As the rightful King of Gondor and all men, the character has deeply-rooted history in Middle-Earth which would make for a fascinating prequel series. For one, thanks to the long lifespans of the Dúnedain, Aragorn was already 87 years old by the time he showed up in the movies, meaning there's a whole lot of adventurin' Middle-Earth's manliest man probably got up to before Sauron got cooking on his plan to rule them all.
As Amazon are developing the series in cooperation with the Tolkien Estate and Trust, publishers HarperCollins, and New Line Cinema (they of the Jackson film trilogies), they will doubtless have access to all the reference materials they might need, possibly including a copy of Appendix A covered in annotations written in Tolkien's own flowing hand.
TheOneRing speculates that, should the series focus at least initially on Aragorn, other characters that could show up include Aragorn's father, Arathorn, as well as Elrond's twin sons, Elladan and Elrohir.
It's worth noting that the rumor specifically states that the first season is going to center on Aragorn.
Of course, there has been no official announcement from Amazon (the series is, after all, still at an early development stage), so we have to treat this as still in the realm of rumour and speculation for now.