The movie District 9 recently came out in mid-August. It's a science-fiction movie directed by Neill Blomkamp and produced by Peter Jackson. Why is this relevant? Well, before the Halo film was put on indefinite hold, Peter Jackson was to be the executive producer and Neill Blomkamp was to be director. Once Fox and Universal pulled out, the two began work on District 9. So, with no Halo movie to see, I consider District 9 a look at what the Halo film would have been. (Minor spoiler warning for District 9.)
First of all, it should be noted that while no actual movie was made, there were live-action shorts used to promote Halo 3. These may be a closer look at a Halo movie than District 9, but District 9 was a full-feature film. Both share some common elements in the direction style, which add credence to the possibility that the Halo film would have been made similarly.
What elements? A strong emphasis is put on realism, without the smooth and stylish appearance of most American science-fiction, and many of the shots are obtained through security cameras and other recordings. District 9 was edited to look like a documentary, and it began and ended with interviews. While Halo wouldn't have been made in quite the same way, simply because of it not being Neill Blomkamp's creation, I can imagine it featuring some edited-in content like UNSC propaganda. The Believe shorts were presented as a series of interviews long after the Great War with veterans of the battles describing what it was like. The timeline makes it so that the exact Believe shorts are unlikely to have been included, but possibly something in a similar style would have been used.
Casting: Since the Halo film was announced, fans have spent time arguing over which actors should play which characters. I believe at one point it was rumored that Denzel Washington would play the Master Chief because he met the film's director for lunch or something like that. Well, I don't know about Denzel, but District 9's cast is made entirely up of unknowns. The cast was limited to only a few main characters, but with a large quantity of minor characters to provide ambiance. The District 9 actors appear to be mostly native African, meeting the setting of the movie to a degree not usually seen in Hollywood. I'm not sure would this would mean for the UNSC, but it might mean we would have seen more of the blended culture elements than are normally seen in Halo.
Content: District 9 has a firm R rating. For those outside the U.S., that means no one under 18 can purchase a ticket without a parent or guardian to show ID. The film earns it through its depiction of violence and oppression. The word "fuck" is thrown around a lot, especially after the film's plot causes the main guy's life to go to hell. The guy is initially very racist and sadistic, which is disturbing even if he doesn't really swear or anything, simply because of his casual attitude to horrible things - I would compare him to a Nazi with a desk job. The plot of Halo is quite different, but this is the kind of stuff the director felt necessary to include in District 9, which can perhaps give hints as to what kind of content would have been in Halo.
The aliens of District 9, not really given a name besides the ethnic slur "prawn", are very different from the Covenant. While the Covenant are a fanatical empire bent on human destruction, the District 9 aliens are a group of refugees who just barely made it to Earth on their liferaft of a spaceship, and they are held in an internment camp underneath the parked ship. This storytelling technique of using aliens to commentate on human racism may indicate that the Covenant would have been placed in a morally ambiguous light instead of just evil. Characters like the Arbiter and 'Vadumee may have been introduced to make the other side appear more sympathetic. The physical appearance of the aliens does resemble the Sangheili to some degree. Perhaps by looking at the District 9 aliens, we catch a glimpse of how the Elites would have looked?
The alien weapons of District 9 are not really like Covenant weapons. They neither look or behave the same way. The small ship used to fly up to the main ship resembles the Pelican in some respects, but not really beyond its VTOL engines. The one real technological connection, in my opinion, is the mecha.
Halo Wars features a mecha unit, the Cyclops, used for building destruction and repair. In the standard American style, the Cyclops mecha is bulky and squat while basically resembling a man. In District 9, the mecha is the creation of the alien race and is thus made to look basically like one of them instead of a human. The District 9 mecha is a lot more agile than the Cyclops, and is used to run around shooting things. They resemble each other in the basic aesthetic style, though. I have to wonder, if Neill Blomkamp likes mechas enough to stick one into District 9, could he have put one into Halo? Had the Halo movie been made, it is possible that it would have defined the Cyclops before Halo Wars.
In conclusion, the District 9 collaboration between Neill Blomkamp and Peter Jackson offers us a look at what the Halo movie might have looked like had it been made. This is largely guesswork, pieced together from common elements. I feel like a paleontologist speculating on dinosaur behavior based on the behavior of their modern-day evolutionary cousins. Well, like the chicken to the T-Rex, District 9 is our closest link to the lost movie.