I want you to imagine something: imagine your favorite video game. Now imagine that they program in a cutscene-to-gameplay ratio of 20-to-1 (for many JRPG fans, this should feel familiar already). Now imagine that, in that gameplay, you control only the movement during non-combat situations. That is, whenever Master Chief is walking down a tunnel by himself, you can control him. But the second a Grunt shows up, the game takes over for you. Now imagine that when controling the character during non-combat situations, you can't stop the character from talking to everyone he gets within 20 feet of. And finally imagine your favorite character played by some live-action star who doesn't really resemble in any way the character you're so used to controling.
That would be a video game movie.
Video game movies don't need to exist. When you play Halo you're already staring at a screen, so why would you want to do that AGAIN for the price of a movie ticket? Besides, odds are the action scenes would be cooler in the game then they would be in the film. Imagine the "give the Covenant back their bomb" scene in Halo 2. Could they really do that in a film and make it look as cool?
So, to close this mini-rant (apologies, all), I believe that now is NOT the time for a Halo movie, or any video game movie for that matter. And that time will probably never come.