To be able to control current Fire TV devices with Amazon's voice assistant, you need to use its accompanying voice remote or a separate Echo speaker. You apparently don't need either with the cube-like device, and since it has a built-in speaker, you'd even hear Alexa's responses even if the TV isn't switched on. The device will reportedly be able to handle 4K HDR video, as well.
Of course, we can't say with 100 percent certainty whether that's truly what the "Fire TV Cube" is. As AFTVNews noted, it's also unclear whether "Fire TV Cube" is an official name or something Amazon decided to use for now, since that's what fans call the device in the leaked image. Your best bet is to sign up to receive more info and hope that the e-commerce giant sends out one with all the juicy details in the near future.
Update: A reader sent us scans of the manual that came with his Amazon Ethernet adapter, and one of the pages showed an illustration of the Fire TV Cube. The ports on the device's back are marked as "Infrared" "HDMI" and "Power," while the micro-USB port is marked "Network," which likely means it can't power the device through the TV -- you'll have to plug it into an outlet. Between the sign-up portal and this page in the manual, we might be seeing the Fire TV Cube very, very soon. [Thanks, P.L.!]