To go on top of the Battlefront 2 idea, there'll be different years from the war.
This would be the only Human VS Human mode, providing units like Marines, ODSTs, and Mark IV armored Spartans IIs. The Insurrection side would have equivalents, even tough guys in heavy armor to match the Spartans since this was before Spartan washouts.
This is what was mentioned above, minus Spartan IVs. Nearly the same as the main games formula.
This takes place during Halo 3, on the Ark. Sangheili are now an ally unit, while you have a Jiralhanae to take its place in the Covenant side. Mostly the same as before, just cosmetic differences.
This takes place during the Requiem campaign. The Swords of Sangheilios are now represented with the classic look, while the Storm has the modern look. Sure the Swords were not there on Requiem, but they did fight against the Storm elsewhere in the galaxy. Prometheans would be a neutral enemy, fighting both sides.
This would be during Halo 5's campaign. It's like Storms Approach, however the enemy roles are reversed as the Storm are the neutral enemy, players are Promethean.
This is during Halo Wars 2 in 2559. Old Human tech vs high advanced Brute tech. Plays like the HvC war, minus Jackals and Drones. Even if the Brutes outnumber you three to one, it is still an even fight.
This final mode, players control the Banished and Flood. Most of the Flood units are AI, but provides constant pressure to the other team.