According to a post on Reddit by a so called tester. As you can see the post was taken down, but don't you worry, took a screen shot before they did, this is what was posted.

•  So the next HUGE overwatch event/update has gone 
   gold, I got to play the new event as a tester and I got 
   some information about it. 
   This event is going to be simply called "Overwatch 
   Anniversary Event" and it's about the same size as the 
   Uprising event. It has a VERY similar PVE mode. This 
   takes place in the Ominc Crisis and you play as young 
   Soldier 76, young Reaper, young Ana and Rein with his 
   fabulous hair. They will be all reusing the old "young 
   skins", but rein will get a new skin where he has that 
   amazing hair. The character "Liao" will be talking with 
   you through the voice chat and giving you directions 
   (possible new hero in the future? we can hope). You will 
   be defending against the omnic forces and you will do 
   an counter attack against them in the end. This will last 
   for 10 waves, and then you will attack. 
   The main menu will have an AMAZING shot of all the 
   heroes posing together. You will be granted a free 
   lootbox (as always), but it will be a normal lootbox. 
   There wont be a new lootbox for this event, because all 
   the new skins and such will be added to the normal 
   boxes. 
   I will list all the new skins below: Reaper = two 
   legendary skins, the other is a re-coloring like the 
   uprising torb one. Sombra = one legendary skin Hanzo 
   = two legendary skins, both different. Bastion = one 
   legendary skin Sym = one legendary skin Tracer = one 
   legendary and one epic skin Orisa = one legendary skin. 
   There will also be new victory poses, highlight intros 
   and emotes. 
   This event also ships with MASSIVE balancing 
   changes, that are already on the ptr, and some that 
   arent. Probably the biggest new change is mercy 
   getting a second use for her ult, which revives her and 
   gives her 50% ult back. 
   Also, everyone who has had overwatch for a year, will 
   get a gold medal to their profile, under their name. 

Needless to say this will be epic if this is true. My god a secondary Ult for Mercy is a game changer! Also, no mention of Doomfist in this event so I guess we'll just keep on waiting. Happy Anniversary Overwatch gamers!