I don't know if this had been discussed somewhere previous to the release of Generations Thunderwing, but I just saw when unpacking the figure that there's a different head on the instructions.
If you check the instructions that are available for download via Hasbro; Thunderwing, the right head is depicted, but not on the instructions that came with Thunderwing.

It is probably a upcoming figure as we seen before on other instructions for other figures.
So people, speculations. Any idea what figure this might be?

Here's a picture that i took from my instructions;

It's most likely the Victory character Black Shadow, who was originally a retool of G1 Thunderwing.

Interesting. I didn't know that about Black Shadow being a retool of Thunderwing.
I really like Thunderwing but a repaint of him as Black Shadow...WOW! Black, silver and gold, that would be a winner for sure

Looking at Swetransformers' custom commissions, I'm actually thinking Energon Galvatron here... the "jet" mode is close, and the horns are close. I dunno.

This was the overhanging fan-speculation when the instruction scans first started surfacing. But comparing the three side to side, the model depicted in the instructions shares too many features with Black Shadow that aren't present on Energon Megatron.

The two designs are quite similar though, so who knows. Maybe Hasbro are intending to use it as a homage to Megatron, seeing as the western demographic is far more likely to be interested in that character, while TakaraTomy might release it as Black Shadow (in which case TakTomy will be getting my money for sure ).
Though seeing how they've thus far been focusing so much on G1/2 continuity characters recently and given the amount of obscure characters they've pulled out for the Decepticons I personally doubt it's going to be Megatron. Only time will tell for sure.