That would've established the droids right up front as a threat to be taken seriously.

It would be a much more interesting opening. Like RedLM said, ANH started out, and right away the story was being told without saying one word. The same could be done with TPM.

Mabey show a little of the battle with the having the Gungans meeting up with the droids with Naboo city scapes in the background.

Have the gungans get wiped out and then show the droids marching in raiding the city.

As shaner said, it would right away establish the droids as a threat.