I am planning on GMing an RP group set in the Halo Universe. Going to use the Rogue Trader rules-set. Anyway I wanted to bounce some ideas around and see which one people though was best.
1 - Our heroes have to take down a group of Spartan 3 who got triggered and have actually joined the Banished as mercenaries because they hate the UNSC. They join an invasion of a Banished fortress world and one thing leads to another before the world gets blown up by Cortana’s shenanigans and our heroes need to escape.
2 - Our heroes wake up in cryo over a mysterious tropical paradise world. Unbeknownst to them it is the home of an entire order of reclusive and zealous Ascetics. This all female warrior caste had isolated itself from the Great Schism and make plans to strike against the UNSC. Basically imagine Sisters of Battle if they were Elites. 
What nobody realizes is that the Forerunner artifacts they worship are actually part of the Soldier factory that the planets crust and atmosphere has simply accumulated on after millions of years.
3 - Our heroes are stationed aboard a Lunar fortress which comes under attack by the Prometheans following Halo 5.