There are a plethora of interesting characters from the Halo novels/comics that could be great additions in-game. The Spartan-II Blue Team is already a popular choice, but there are plenty of other character choices that seem to get overlooked.
Ignatio Delgado (The Cole Protocol): His background living among the Insurrectionists on The Rubble could not only bring more information on the rebels, but it could make the fight against them personal for him, and loyalties could be tested. Of course, this is assuming the Innies even appear in-game. His assumedly Latino heritage would also simply add more diversity.
Major John Smith (Halo Evolutions: The Mona Lisa): He performed illegal experiments with the Flood aboard the prison ship Mona Lisa. He may have very well been executed for his crimes. Or ONI could have looked the other way due to the information on the Flood he may have gathered.
Lieutenant Parisa (Halo Evolutions: Palace Hotel): Parisa was a childhood friend of John’s before he was abducted. It’s implied that they were very close. Her inclusion could bring about more of John’s suppressed/forgotten memories. Whether this leads to positive or negative effects is up for debate.
Spartan-II Gray Team (Cole Protocol): A Spartan team that operates behind enemy lines. They are some of the few Spartan-IIs that MAY still be alive.
Spartan-II Black Team (Halo Evolutions: Blunt Instruments, Bloodlines): Same as Gray Team.
Chief Mendez (The Fall of Reach, Ghosts of Onyx, Glasslands): The trainer of the Spartan IIs and IIIs. An ally and friend to the Spartans. Has a “complicated” attitude toward Halsey. If he were to appear, it couldn’t be around John, as it was mentioned that they never saw each other again.
Staff Sergeant Byrne (Contact Harvest): an old friend of Johnson’s. Years of experience fighting rebels. Possibly a Spartan-I like Johnson (just my speculation).
The Iso-Didact (Cryptum, Primordium, Silentium): basically the new Didact. Ultimate fate unknown.
Chant to Green (Silentium): Assumes the title of Lifeshaper after the Librarian’s death. Chant to Green is, essentially, the new Librarian. Ultimate fate unknown.
There are probably a lot more characters, but I don’t want this post to be too long. Feel free to add to the list.