An image of a second figure has been tweeted - Nacelle - still not giving us any modes we haven't already seen. (there are four figures that have only had one mode revealed so far, so now would be a good time to reveal them)
It does include artwork of the character though, which should be its biocard image.

Apparently, Pete from FunPub has mentioned somewhere (I can't find it to reference) that the bonus figure is a Generations Legends figure (the ones that come with a "Targetmaster" partner)... which would mean a redeco of either - Optimus, Bumblebee, Starscream, Megatron, Swerve, Cosmos, Skrapnel, or Tailgate.
Could be anything from any of those moulds, since only one of this year's main six was anything obvious/expected (Gen2 Starscream).