Natascha! - Update
Update - The achievement descriptions are up, and they pretty much confirm everyone's favourite guess for weapon upgrade three, the "Sandvich". The Konspicuous Konsumption achievement requires that you "eat one hundred sandviches," which makes them seem a pretty solid bet.
Sasha has a playmate. Valve's cunning trick of stringing out their update announcement to forty-seven days is working, as proven by today's Heavy Update Post: Natascha is the new member of the familiy, the second revealed unlockable weapon, and she's very pretty.
This is a rather fascinating new weapon. When minigun Natascha's bullets enemies hit a player, they will slow them down, just for a moment. A slightly weaker weapon (25% less), she's not designed for face-to-face combat, but rather dealing those crowds of weakling retreating opponents, who will find themselves forced to either find a hiding place, or turn and face you, "mano-a-tiny-itty-bitty-mano."