This was revealed during an interview with Eurogamer where Overwatch’s director Jeff Kaplan confirmed that the hero revealed at the end was Echo, and that she is expected to be a playable character in the future. In fact according to Kaplan, the double hero reveal is only the tip of the iceberg as Blizzard has at least six new heroes planned for the game.
However if you were hoping to see them introduced anytime soon, you could be in for a wait as the reveal is expected to take place over the course of the new couple of years. According to Kaplan, “We have our next six heroes planned out right now which, for us, is years. She, tentatively, is one of those heroes.”
That being said while it might be a while before we can play as Echo, Kaplan has stated that Ashe’s arrival on the PTR should be soon (possibly even today), so players should keep an eye out for her if they’re interested in seeing what she can do.