So i am trying something different. I have been GM for a campaign of Only War for a few months now, working through the Final Testament campaign. During game play i joked about crossing Only War with another game from a different universe, for example, Star Craft or Halo. My crew went NUTS over the idea and are pleading with me to make up a campaign crossing Only War with Halo, using Only War as the main game system. I have managed to find a made up copy of a Halo RPG based after Halo 2 on Google, and it shouldn't be too had to convert over to the Only War system.

My original idea to justify the cross over is simply to make it that a UNSC fleet or single ship, while traveling through slip space, experience some form of turbulence due to accidentally entering a warp storm and, due to an emergency stop, shows up in an alternate universe, that alternate universe being the Warhammer 40k universe.

Does anyone have any input that could help me in creating a decent solid story line that will hold in game play? the players would be playing as Imperial Guard, they are currently Elysian Drop Troops. I may have 1 new player join as a UNSC Marine/ODST since i somewhat would like the 2 factions working together in the campaign.

To be honest, this would probably work better in a high-level Dark Heresy/Ascension or a Rogue Trader type game, or maybe Deathwatch or Black Crusade - after all, even a single UNSC ship would be a technological bonanza for the Imperium - safe and reliable non-warp FTL, tech that they know how to produce, even FTL communications, a significant amount of scientific knowledge in the ship's computer that the Imperium doesn't have or has lost. Assuming, of course, that they get cleared by the Inquisition/AdMech investigation that will be an extremely high priority.

Unless the UNSC ship and crew gets removed (either go back home or utterly destroyed) without leaving anything of value behind, and fairly quickly, this would ultimately be a setting-altering event.

The UNSC still knows and understands its tech, and hasn't lost the drive to invent or reverse engineer technology from outside sources.

Depends on exactly when in the Halo-verse timeline they're from, and in just how much force, too.

what if i were to bring 40k into the Halo universe?

Every player at the table argues over who get's to be Master Chief?

Now for me I wouldn't mind being a gaurdsman from the planet Helgast:

We are currently big involved in changing the Star Wars: Age of Rebellion Rules to be used for Only War.

Some kind if the other way round.

Every player at the table argues over who get's to be Master Chief?

the campaign i am running only allows players to go as high as ODSTs. lol

as for Helgast, it could work. i have spent 2-3 weeks converting a whole heap of stuff over from the Halo Universe. weapons, gear, vehicals and enemy types. i am going Pre Halo 4 for the story line.