Overwatch game director Jeff Kaplan announced some "big changes coming for Mercy" as well as other heroes, including D.Va, in a new developer update video, which you can watch below. All the changes noted are currently available to test in the Overwatch PTR.
Mercy’s ultimate ability, Resurrect — which currently lets Mercy revive multiple dead teammates within range — will become a common second ability next to Guardian Angel, with some vital tweaks. The revamped Resurrect will become a single-target revival ability that relies on "a rather long cooldown."
The replacement for Mercy’s Ultimate is called Valkyrie. When activated, it will allow Mercy to fly freely and buff all of her current abilities: extending the range for Guardian Angel and her healing and damage boost beams, allowing her beams to connect with multiple targets at once, and increasing the fire rate and damage of her Caduceus Blaster, for all the "Battle Mercys" out there.
Source: IGN PC Articles
GameUP24 