Amazon.com has a solicitation for a TF:A junior novelization of two episodes, and the cover of one shows one of Megatron's designs in the series.
In addition, a proto-type of the upcoming Megatron toy has surfaced in an auction on e-bay. The pictures have since been taken down, but they have been archived here.
We know a bit about these designs.
These two are both thought to represent Megatron in two different stages on the series. The toy reflects his design at the start of the show, and the one shown on Amazon.com to be a form he adopts later in the series.
Source:
Transformersanimated.com,
Amazon.com