Update: Courtesy of a press release from Blizzard, it turns out this will indeed be a return to Uprising:
"The Overwatch Uprising brawl returns Tuesday, April 10th. This seasonal event turns back the clock, giving you the chance to relive a pivotal moment in Overwatch history. Explore the stories behind some of your favorite heroes, unlock pieces of the past with over 100 declassified Loot Box items, or suit up with your friends and join Tracer on her very first mission in the Uprising cooperative brawl."
Via a cryptic tweet, it looks like Blizzard is either giving players access to a previous special event or a brand new mission. On April 10, "Mission files unlock."
Initiating archive declassification…— Overwatch (@PlayOverwatch) March 29, 2018
Mission files unlock: April 10.
RETWEET to confirm. pic.twitter.com/cTevQ8M5CE
In the attached video, only the King's Row Uprising is teased. In the Overwatch Gamepedia, the King's Row Uprising is described as follows:
Seven years before the present day, on the eve of a historic peace accord, the terrorist group Null Sector launched an uprising in King's Row, causing London to go up in flames. Prior to this, the King's Row Times had reported on growing unrest in London stemming from Null Sector's activities. They took Mayor Nandah, Mondatta, and a hundred other prisoners captive in a guarded power plant. 27 days into the uprising, the British government forbade Overwatch from intervening, even while thousands in London needed medical aid. In truth, Blackwatch agent Jesse McCree was already on the ground, and had scouted the area. Despite the gag order, Commander Jack Morrison dispatched four Overwatch agents—Tracer, Mercy, Reinhardt, and Torbjörn—to put an end to the crisis.
By the 28th day of the uprising, hundreds were dead, and thousands were injured. McCree reported the existence of Null Sector fortifications, and that they had complete tactical control. The team fought their way through the streets, even as hordes of Null Sector omnics tried to bar their way. The events that transpired were classified within Overwatch's archives.
The previous special Overwatch event took players back to the uprising, which is also Tracer's very first mission. She, Torbjorn, Mercy, and Reinhardt were playable but there was also an additional mode that allowed you to choose whatever hero you wanted.
In the video above, there's are a couple glitches that may allude to an entirely new mission being declassified. Overwatch has hosted a collection of other limited events and it is possible they'll all be available on April 10, 2018, but stay tuned to Shacknews for further updates.