After posting a short clip teasing content related to the origins of Tracer, and King's Row, Blizzard have released a comic revealing what some of that content might be. The comic, titled "Uprising" details Tracer's introduction into the Overwatch team during an uprising of rebel Omnics. The comic is set seven years before the game's current events, during a time where the Blackwatch was still active. As such we see alternate looks for many of the game's characters.
Tracer can be see in two separate outfits, a training outfit and an Overwatch uniform which we also see for Reinhardt, Mercy and Torbjorn. McCree and Genji appear in their Blackwatch uniforms and we see what looks like an alternate version of Bastion. Mercy and Winston appear in lab coats, although it's unlikely these will end up being skins. All of these looks can be sen bellow
In addition to skins, the comic may be hinting at some sort of uprising event as we saw with Halloween's "Dr. Junkenstein's revenge". A horde mode with players battling waves of rebel omnics is entirely possible, however this is all speculation since neither and event nor new skins have been officially confirmed to be part of the April 12th announcement.
The comic can be read here: http://comic.playoverwatch.com/en-us/tracer-uprising