I'd be happy simply with a Grey/Silver iteration of a Tank (His G2 Alt-mode), much like Megatron is depicted in the 'Transformers: Legends' mobile game:
I realise what works on a canvas doesn't necessarily translate to a 3 dimensional, tangible assembly of plastic - But if Hasui's work with the Transformers: Animated line is anything to go by, I think this look is totally achievable; the Bot mode looks almost exactly like the G1 show model and the Alt mode avoids an immense amount of legal hurdles.
