If there is a will there is a way, with the best technology (computers) out there - there is little doubt that it could be done if they wanted. If they do a Death Star sized Unicron/Primus (but with the alien design to it). It's obvious not going to be realistic... but who cares, its Transformers and its a science fiction flick.
I've seen the interview reguarding the producer, and his opinion that it would mean less screen time for the humans and make it less realistic... well obviously it already passed the relm of realistic and its Transformers... and that its weakness that it uses pointless things such as Sam going to College, etc. We don't even get to see the Transformers interact with humans much at all, so. With Unicron and an intergallactic battle, it would force the issue of having a good plot with uber-science fiction elements (hence Independence Day). Even Shia LaBeouf hopes there is a intergallactic battle sequence, he said in TF1 you were introduced to Megatron, in TF2 there was a bigger threat (the Fallen and Megatron) - TF3 should follow up on that.If he doesnt reach earth then it will have to pass in deep space and that goes off the point as in M Bay wants a human-Robot interation.
I agree, but I think it needs to have a really good story. A little mythology of Primus and Unicron would be really cool as well.All in all, im not against Unicron but i would like it done right.