Posted just now
https://twitter.com/PlayOverwatch/status/750736158624215040
Those who have been paying attention to theories say Sombre
Sombra*
Sombra is a healing sniper
this is a healing sniper rifle
I'd say thats a pretty good guess
The theory is that she's a long-range healer. Hit an ally? Heal in some way (I think the rumours say that it's like Zenyatta's harmony orb, but can be applied to multiple people for a set amount of time). If you heat an enemy, it's some sort of debuff or DoT. Hard to tell. Not sure what other skills will be.
Lore wise, she's suspected to be past allies of Reaper and 76. She MAY BE Pharah's mother, Ana. Her name means Shadow in spanish, but which side she's allied on is anyone's guess. However, there is a reaper line where he says "Where's Sombra when you need her" - usually said against a sniper.
There's a few theories too about how she is portrayed in the game. She may be a downloaded consciousness from Athena (where she died and her mind was uploaded to Athena and all that jazz), put into an omnic body (Which may make her un affliated with talon, depending on who she met.) There's a few images of an omnic looking headpiece, but it's unconfirmed if it's a full head, or just a sniper mask (like, and advanced widowmaker helmet) . There's a couple places where some HUD elements that are being attributed to sombra support the mask-theory.
... and she has a badass cape, hood and cloak. As a primary support player, I'm very interested in seeing/hearing more.
EDIT: Forgot to add that the other-other rumour (based on the above skematic) is that you'll shoot little 76 biotic fields, rather than the thing I mentioned above.