if Microsoft and Nintendo REALLY wanted to, they could make a story where Samus runs into a Flood infested planet where one of the SPARTANs that went MIA is still stranded there surviving/fighing them off. You could make it like that one GBA metroid game where shes hunting that one alien dude who copies themselves into Samus clone. But instead of a clone you could just have them be the "missing" spartan girl that Samus is (not knowing) looking for.
I'd buy it.
I've read the books, and it wouldnt be farfetched at all for one or two spartans to get their own sort of side story about him or her surviving on an alien planet after being deserted during an attack.
You could even use Nicole from that DOA4 article.
http://www.gamespot.com/news/6141230.html
Have her stranded on a neutral planet that the flood/covenant have overrun because a large tomb held some rediculous amount of power. Since a small human fleet was ambushed there 5-10 someodd years ago no one ever sent anyone there to explore/check for survivors.
Enter Samus, who's doing a little intergalactic patrol and happens to fall upon this huge beautifull planet. You see her catch a glimpse of Master Chick drinking water outside a cave, then runs away. There you could have Samus get ambushed from behind by a few flood / metroid aliens. Her little ship crashes, so she needs some sort of power source to get it up and running again. In order to do that, she needs to enter those caves/tombs to get out.
Inside she catches glimpses of Master Chick but always gets interuppted by a horde of Flood/Elite/Metroidlians. She solves puzzles, gets through tombs, etc etc. Just like any other Metroid game. At the end, she thinks that the Master Chick she's following around is jsut another metroid alien (like in the GBA Metroid) that cloned itself into her.
You could end the game with Samus finally catching up to an already fighting Master Chick, and have them duke it out. Then income some more covenent/flood forcing them to team up and finally talk to one another and work together to get off the planet.
Now you could even spread this over 2 games. One for 360, and one for Wii. The 360 version would play just like any other Halo game except you can switch the levels into different zones that Master Chick tryes to clear of the flood. Since theres an already ambushed cargo ship that she landed with, theres just enough ammunition to handle the jobs. (added with the covenant weapons/ammunition) Towards the middle of the story she realizes shes being followed by another type of alien. You then have her catch small glimpses of samus killing off flood but never make it too obvious.
Since the 2 games will play completely differently than the other, both Master Chief and Samus will stay exclusive to their systems, and people would eat up both titles to get the full story.