I love Halo. So. Much. Been playing since day 1 back in 2001, and I have wanted a movie/TV series ever since (I was SO pumped for the Peter Jackson movie, and then for the Neill Blomkamp film), but I think there could potentially be a few problems.
First, the Chief is a mainly silent protagonist. This works well in a video game because we can project ourselves (our voices, thoughts, looks, etc...) onto him. Similar to Link in the Zelda games (the last one I played was Ocarina of Time, so please correct me if I'm wrong here). Think about how boring a movie would be if your main character doesn't do any talking.
Second, with pretty much all of the media content that MS/Bungie/343i has put out, INCLUDING marketing material for the games (but excluding the 3D animated stuff), the stories have all skirted AROUND the Chief, and only show him in glimpses or something similar (I'm looking at you, Forward Unto Dawn). Having read the Halo books in my younger days, I want to see a Halo series/film ABOUT Master Chief. Yes, there are side characters (Cortana, Keyes, Sgt Johnson, etc...), but Chief needs to be the focus, hands down.
Third, what story do you tell? Do you backup and start with Reach? You obviously need to show the backstory about the SPARTAN program, their training, how they are the loners in the UNSC, the pariahs. Do you start the series/film where Halo: CE starts, and jump straight into the middle of the action? I think a mini-series type of show would do well in the Halo-verse, as that gives you more time to get to know the characters, to make you care about them.
Like I said, I LOVE Halo. I just hope they aren't doing this as a cash-grab and then screwing all of the fans over.
Do it right, or don't do it at all.
SB