So I just completed my fifth playthrough of ODST, and I'm sure of some things I want to see carry over from ODST to Reach, campaign-wise.
1. ODST's FOV. Nothing really to say other than I like it more than Halo 3's.
2. No dual-wielding. Don't try to gimp other weapons just because it makes sense that you can hold one of them in each hand and fire both at the same time. Bungie, you've already done it with the Needler, now just do it for every other weapon.
3. Retain the gameplay idea that all weapons should be balanced so that no weapon is useful in all situations. This is probably the aspect I most enjoyed about ODST's gameplay. It wasn't "use X weapon for close quarters and Y weapon for medium and long-range encounters." The player was forced to think about each situation, how best to use each weapon in an encounter, and which weapon (or both of them) to use on certain Covenant. This was because for one reason or another, all the weapons in ODST weren't effective for every single type of encounter. It was "Okay, I don't have enough ammo of X weapon/X weapon isn't effective until a certain enemy is in a certain condition, so let me use Y weapon on enemies 1-3, and a combination of the two on enemy 4." To put it differently, "I'll use X weapon on enemy types 1-3, and Y weapon on enemy types 4 and 5." Of course, you could always run out of ammo of X weapon in a certain encounter, in which case you have to pick up a weapon from a fallen enemy. However, due to the balancing of the weapons, you still have to think about how best to use the weapon you just picked up.
It can probably be argued that point three is present in Halo 2/3, but I don't think it can be argued that it's nowhere near as prevalent as in ODST.
4. Give me another open-world area. I like the idea of Mombasa Streets. I thought it provided that evolutionary step that Halo FPS games have needed for awhile. The idea can be improved upon in several ways (not-so-samey all around, more lively, make it more interesting to explore). All of Mombasa Streets' shortcomings were probably due to the size of ODST's development team.
5. Audio logs. While the story of the audio logs in ODST wasn't as good as the story found in Halo 3's terminals, the presentation and implementation of the audio logs was far better. It's a fantastic way to tell a story, and it's entirely optional, so it doesn't burden the player. Keep 'em coming, Bungie.
I'll add more if I think of anything.