Halo 3: ODST is one of my favourite Halo games ever, joint third on my list with Halo CE/PC (Halo 2 and 3 are the two above it). It added a real good sense of isolation and mystery, as well as having an expansive open-world map, a feature unique among Halo games.
I am both disappointed and suprised that there has not been a sequel or prequel for it, as it was extremely successful and fun to play. There is also a perfect place for a prequel; New Alexandria during the Fall of Reach. Buck was present, and what's more, it is perfect for another game; a razed city with a lone ODST in it. You could even be saved by your own Spartan from Halo: Reach!
I've created a mockup of what it could look like:
So what do you think? Would you like more ODST Halo games? Why/why not?