I went gun mostly for nostalgia. The argument about guns being a passive tool fired by someone else never bothered me. Rather Megatron transforming into a gun was a signal that things were getting very real.
I never believed there was a good Megatron tank until the Combiner Wars Leader. For the first time it combined a tank mode with a G1 aesthetic which showed the concept could work. For me the old green and purple tanks were like Superman showing up in green and yellow.
I don't think Megatron should ever be an aircraft. That is Starscream's role and messes with the pair's dynamic.