As much as I love ODST, I dont think 343 understands what made that game great. Plus it's timing when it was released was just perfect. You won't be able to get that again with a sequel. With the crazy amount of reworking they have to do with the current Halo story, where would that narrative weight from a ODST sequel come from? Why even have ODST's anymore when anyone who qualifies for a ODST can now also be a spartan? I just dont think it would work.What if it were a prequel game set before 343 games? ODST was set before 3, this one could be set just after 3 because unless you read books there's a number of years that players miss in between the events of 3 and 4
Well, heres how that's a problem IMO. What made ODST work so well is that for 3 games, you've been the chief. You've survived everything thrown at you, blown up everything in your path, always won, had the most cinematic moments in history etc. Even though humanity was losing, you as the player never saw that loss really, never felt like you were losing. Then ODST happens and reminds you that, we weren't actually winning. It takes that sense of invincibility away from the player. That most of humanity were just regular people, not Spartans who could do it all. Its mood is different, its setting is different, its theme and message is different. Thats what makes it so memorable. To just make a prequel set in the same time frame, while the game could mechanically be top notch and the art fantastic, you're not going to get that same weight again. We can see it with Reach. Reach was great. I loved reach. My longest time online has been with reach, and that final scene where you hold out as long as you can is great. But you already knew how it ended, and you've already felt that weight of losing a war before reach came out. Which makes Reaches story, not hit as hard as ODSTs. (Of course, if the releases of the games were switched my opinion might be flipped, who knows?)
But as far as lore goes, I see another problem. What could you do between 3 and 4 for a ODST game? Well, more than likely you'd either be just be going to planets with covenant hold outs pre J'ul's takeover, or you'd be sent covertly by ONI on special missions for say clean up of possible flood left alive or maybe even assassination missions. Neither of which is really fitting with the feel of a halo game. It's more like a COD black ops type of feel. At best, I could see playing as a ODST who's recruited for the first wave of spartan fours, but since it's not pubic knowledge at the time, you'd be secluded on Infinity. It's possible to make a good game in that isolation, secrecy, and having ONI having complete control of your life, hell you could even get punished in game if you don't follow ONI's commands that you find disagreeable to. But a game like that would be very anti ONI and 343 has never lambasted ONI and seems to never point a mirror back onto ONIs crimes whenever they decry the crimes done from the Spartan II program.