Nothing is true. Everything is permitted. Understand the meaning of these words. It matters not how we complete our task, only that it's done.
1. Have Dr. Halsey make an appearance - She's the entire reason the Spartans came into being in the first place, and Cortana was developed from her. She's a critical part of this universe's story and she's around during the fall of Reach, so it'd make sense to have her show up in the story. On a side note, she'd be unique to have around because as the books show, she has that motherly part of her and is the only one that calls the Spartans by their actual names. In the midst of an epic battle where everyone else only treats them like war machines, Halsey's presence would uniquely humanize the Spartans and remind players that these are people that are fighting and dying for the greater good.
2. Play out the drama - On that note, the story should really play out the emotional drama. The sacrifice the Spartans are making, the hopelessness of the Marines and all the death and destruction...lots of potential for heavy material. The UNSC loses all the time to the Covenant throughout the full story of Halo and here there's the chance to show that other side, where Master Chief doesn't save the day and the battle is doomed.**
3. Show everything Spartans are made of - Their heroism, their unquestioning focus, their sheer skill and combined ability as a group and the inspiration they inspire make Spartans the stuff of legend. At the final stand many of them make in the story, it's only fitting these elements be spotlighted. Further, the differences in skill between the Spartans should be shown as well, likely through gameplay.
4. Have Master Chief make an appearance - While he isn't part of the planetside battle, Chief is there along with everyone else before they get into the fight. We haven't had the opportunity to SEE him interact with the other Spartans inside a Halo game, and considering he was the leader among them it would be great to see that dynamic actualized, to see him give orders and establish that feel of what he was like as part of a team before the trilogy.
5. Explore Reach's Forerunner connection - As shown in First Strike, Reach's human presence isn't the only reason the Covenant are interested in the planet. Forerunner secrets are buried beneath the surface of the planet, and are noticably different from those seen anywhere else. Having the presence of these acknowledged, or further play some role in the story, would be great to see. The mysteries of the Forerunner are the key reason the humans and Covenant are at war in the first place, so integrating that element would provide some needed perspective for the story.
6. Expand the arsenal and equipment - Spartans use everything available throughout the story of Halo to succeed. Besides what we've been able to use in gameplay before, there's much more weaponry and tactical devices we haven't seen used. From combat knives to the Jackal's portable shields, from custom sniper scopes to specialty explosives, the possibilities for new gameplay experiences are deep. Bringing these ideas to reality is necessary and would well compliment the new experience of being among multiple Spartans.
7. Have the Engineers present in game - In function and attitude, the Engineers are starkly different than all the other species composing the Covenant. In spite of their unique nature and vital role to the Covenant itself, we haven't been able to experience their presense in game. Running into these non-violent aliens simply going about their repairs would make for a completely different feel as a Covenant encounter, especially in the context of killing them. This species needs to be encountered sometime within a Halo FPS experience, and if they aren't to appear in ODST, then they should be in Reach.
8. Spartan flashbacks - Reach is an ironic setting if we are indeed going to be among multiple Spartans, as it was the very planet where they became what they are. As the Spartans engage in various missions throughout the game, it would be interesting to see them recognize a location from years earlier during their training and have flashbacks showing those moments. We haven't been able to see the experiences of any of the Spartans as children before, and Reach is the ideal of setting to bring these experiences to a Halo game.
Of course, everything's speculation for now, but these are ideas I think would work well with what we already know.