My idea is, you play as a soldier (Covenant/ UNSC).
You play in first person, the twist is that you don't play a notable character, or make commands in real time like in Halo Wars, because it's an FPS. You develop your own character, that you imagine yourself being in the Halo universe.
To begin a game you must first select a species (eg. Human/ Sanghelli). Skills are aquired by performing tasks, and completing missions. Which will help you aquire new gear, training, that can then lead to bigger and better events in the Halo universe. For example, play long enough as a Marine, fighting in battles, you'll eventually become an ODST. Then you'll get ODST missions and gear. If you decide that vehicles are you preference of battle, you'll get more vehicle based missions and gear (EVA helmets, etc.)
This can definatley shape more online players into different roles, where they can contribute to bigger battles and skirmishes, away from single player modes. Instead of your standard multiplayer, where every1 is equal (spartans vs. spartans).
Single player modes may lead to bigger discoveries. Uncovering forerunner artifacts, for example, getting infected by the flood.
Does any of this appeal to any1?