I agree, while it wouldn't be "game" canon, I think it would be interesting to give different styles of Battle Rifles to Marines and ODST's and then they wouldn't all have the "same" weapon.
Similar to what they did in Republic Commando with the sniper attachment, the rocket attachment, the machine gun attachment for the normal dc-17 for the Commando's, and then they just used a pistol for close up combat, and used their Gauntlet knives as their melee sidearm....