I think an ODST 2 could be possible, though unlikely due to mixed reception of the previous game. To improve upon Halo 3: ODST, 343i would have to make multiple changes and upgrades to it. For starters, if ODST 2 were a larger project (like Halo: Reach and Halo 4), it would require Firefight matchmaking, its own competitive multiplayer along with multiplayer maps, new weapons, and new enemies like Promethean and the Elites.
It should also make some big adjustments to the gameplay. In Halo 3: ODST, the ODSTs played far too similarly to SPARTANs; they could swing gravity hammers, detach turrets, and down enelies with just a couple of melees. While playing like a SPARTAN is the conventional Halo formula, it just isn’t realistic or immersive when playing as ODSTs, and I’m sure 343i can make you feel like an ODST while also making it seem like Halo. This can be done. Before Halo: Reach, the Elites practically played the same as SPARTANs. But now there is gameplay differences between Elite and SPARTAN models.
The same can be done for ODSTs, simply by changing their size, slowing them down, and reducing the amount of damage when they melee enemies. One of the most neglected features in Halo 3: ODST was stealth gameplay. Halo 3: ODST has may settings and themes which practically begged for stealth gameplay, yet hardly any of it was incorporated into the game. If stealth gameplay was emphasized in ODST 2, to the point were being spotted by the enemy would likely mean death, it would make the gameplay much more interesting and realistic.
One of the strongest things about Halo 3: ODST was its extravagant characters, deep themes, and rich setting. It was a much more personal ‘human’ story, and an ODST 2 should follow in its predecessors foot-steps by having another strong cast of characters, more deep themes, and another wonderfully rich setting. One of the things that exemplifies this is Halo 3: ODST’s hub-world New Mombasa streets, the place where the player would roam around and activate the game’s missions. It was dark, lonely, and interesting experience. ODST 2 should have its own version of a hub-world for the player to explore.
I’d also be delighted to see the Audio logs (Sadie’s story) return, because they gave us so much back story about the city, the people who lived in it, and the game’s main storyline. They shouldn’t be text-based or CGI, but actual audio logs similar to a radio drama. The cast, the audio logs, the story, the themes, and world were what fufilled Halo 3: ODST’s film noir so rich and rewarding. And as such, I think they should all return in some form in ODST 2 and be maximized to their full potential. I doubt an ODST 2 will ever be made, but these are some of the things I would want to be in it.