The line "Find the freak", along with the repeateded showing of the guy who accidentally opened up a can of alien Raid in his own face leads me to believe that they have a "Capture the MacGuffin Man" side plot going on. He's repeatedly seen under unusual circumstances, not limited to medical tests conducted without his consent by some random Government Agency (may or may not be the FBI), piloting a car-sized alien mecha, getting randomly punched by an alien, getting kidnapped by aliens and subjected to some sort of painful treatment (possibly torture, brain-washing, or a more literal "conversion"), and firing an energy weapon (possibly alien). The likelihood of him being the protagonist is immense.
On another note, it appears that the aliens were using the 9th District to hide some nefarious purpose. Possibly to construct the giant robots and the plane-thing seen in the trailer, in order to get away with their ships safely? I base this on the line, "Nobody really knew what this place was". Although the most likely place is the titular district 9, the ambiguity of this statement can make it apply to any number of other things, namely a covert government agency, or some sort of precursor ruins somewhere on the planet, though both seem unlikely.
Next, the line "It's going to be quick. It's going to be clean. Best of all, it's going to be
quiet" suggest something is happening behind the scenes throughout the movie. As the last part of it is played, we see we see a man with a muscular stature (presumably military) talking to a balding man in very formal attire (presumably a politician). To me, this can mean one of two things:
1) The military man is informing the politician that he has the best darned plan in the world to rip off the aliens that
can't possibly go wrong. This is supported because of the sympathetic light the aliens are shown in (they live in a slum. Hard not to pity them for that).
2) The military man is informing the politician that the aliens can take over the world in a moments notice and there's very little that can be done to respond to such a threat, if it was to occur.
Finally, there is the crests that appear during the trailer. It is seen at 1:32 at looks like a red triangle split down the center. I may be looking into this too deeply, but I think that it has plot significance. Of course, it only has significance in relation to other things, because that symbol itself is meaningless to us. Perhaps the symbol is displayed on the side of one of the ships that the bug aliens have hidden in the dirt, and the relevant discovery is not the symbol, but the ship. Or it could be an
arc word.