It certainly does seem like a natural progression for Echo to be able to control Fire TV. Since they are both Amazon products, it should be convenient for the Echo engineers and Fire TV engineers to get together and make it work.
Even with Alexa living inside Fire TV now, it still can't control Fire TV. There are only a few buttons on the Fire TV remote and allowing voice commands to control those buttons would be a great start.
So far, Amazon has been oddly silent about Alexa-controlled Fire TV. They have not said yes, no, maybe, or given any hints if they're working on that at all.
If I had to guess, I'd say they're working on Inferno TV, which will replace everything they have made so far. It would be Echo, Fire TV, Dash button, Fire Phone, Fire Tablet all rolled into one unit that connects to your TV. And it would operate regardless of whether your TV is on or off. (btw, I'm half way joking)