We've known changes were coming to D.Va for a while now; her Defense Matrix is getting mega-nerf'd, she's getting rockets. Essentially, D.Va's being pivoted to be a more offense-driven Tank hero. (Which makes sense given that's how most people play her, myself included.) What we didn't know was that big changes are headed for Mercy too, as Overwatch director Jeff Kaplan revealed in today's Developer Update.
Perhaps the biggest change coming is that Mercy's getting a new Ultimate. Her big Rez instead is going to be a second ability used only for single allies on a cooldown in order to discourage the strategy of players telling a Mercy to hide until an entire team gets wiped out, only for her to glide in and resurrect the whole team. Her new Ultimate is an ability called Valkyrie, which Kaplan describes as making everything Mercy does better. She flies in the air while it's active, her healing and boost beam is much faster and more effective, even her little pistol does more damage.
The drastic rework of Mercy is available to play for PC owners of Overwatch in the current PTR (Public Test Realm). There are other changes coming to heroes in the PTR too. D.Va's much chatted about changes will be available to test out, including her lesser Defense Matrix, the ability to shoot her Fusion Cannons while boosting, and her new secondary ability: Micro Missiles. Also Reinhardt's getting a teensy change—you'll now be able to rotate the camera when his barrier shield is up, giving the player more situational awareness. You can read the full PTR notes on Blizzard's blog.
This isn't the first time a hero's changed so drastically. Symmetra, perhaps, underwent the most intense changes of all the heroes aside from the usual balance and cooldown tweaks (R.I.P. Roadhog's hook), adding a new second ability and a second Ultimate entirely.