To seriously consider the possibility that God was a visitor from another planet, we have first to abandon completely all pre-conceived notions taught to us by religion. I see the bible, fundamentally, as a history book, and if, just for arguments sake, we assume that it is, we have to ask 'who was this being the ancients worshipped as a God?' Personally, I don't believe the authors invented him. I believe that he, and all the other Gods of ancient literature, existed, but that the people who wrote the accounts had no way of understanding the advanced technology they witnessed, or indeed, technology of any description, and therefore viewed these 'magical' beings as Gods.
Perigra, you say there's no evidence, but there is so much on this earth that we can't explain. The bible,and other ancient texts, speak of flying machines, and the ancients drew depictions of flying machines. If the authors had never seen such things, and had no conception of such things, where did they get their ideas? And just take a look at the megalithic structures dotted around the earth, constructed with stones of unimaginable size and weight. I would say there is a plethora of evidence. Sadly, since we're all convinced that our 'time' is the only real time, and therefore the idea of people from other planets visiting earth in the distant past must be impossible (or even if we do consider it, won't speak out for fear of ridicule) we don't seriously consider it. I don't believe it is impossible - in fact I think it's highly probable. I've searched, and I can find no other logical explanation.