Guild Wars 2 lead designer Eric Flannum has left ArenaNet to join Amazon Game Studios. He's working on a PC game which will utilize "the latest technology to explore third person, competitive, melee-oriented action gameplay in an entirely new light". That's presumably the same project Amazon was hiring for back in June.
That quote above comes from a LinkedIn post by Flannum, where he's listed as a creative director at Amazon Game Studios. He lists a handful of employment opportunities at the studio and claims the current team has talent from "projects like Guild Wars, Forza, Halo, Starcraft, Half Life 2, Left for Dead, Dota, Infamous, Sunset Overdrive, Destiny, and Shadow of Mordor."
The studio does indeed boast some interesting talent, including Portal's Kim Swift. Far Cry 2 mastermind Clint Hocking was employed in 2014, but left in August. That apparently prompted two other designers – principal game designer Jonathan Hamel and director of narrative design Eric Nylund – to leave too.
There's still been no public announcement regarding Amazon's mysterious project(s), but whatever happens, it'll be interesting to see what Amazon brings to the table – especially with its apparent PC focus.