All that is needed for G series is...
I personally would welcome a 400 mm f/4.5, and I know that others would be interested. A good quality TC x2, together with a 200 mm f/2.8 would be almost equally good, but probably more expensive.
And no, I don't think that a 400 mm f/5.6 would be too big for this system. The GH-3 has a size just adequate, and for example the Canon 100-400 is quite compact (for what it is) and just over 1 kg.
And don't forget all the photographers that are using MFT with legacy lenses of 400 mm +, quite big in some cases, and only MF.