Great mod Idea!
Just some idea's
Normal Marine sprites > Halo marines Sprites
Add new ships > All UNSC and Covernant Ships, also you could add Flood as a enemy like SPAZ (Maby add Forerunner ships as secret ships)
Mission Idea:
*Perhaps add a Halo ring in the backround for the mission*
You need to go in and investigate a SOS beacon, when your ships arrive outside of the ring, you see a group of (perhaps ally) UNSC/Covernant ships, around 3 to 6 fighters protecting a capital perhaps, being attacked by around 10 to 18 flood ships (any type, your choice) and if you sucseed at protecting the capital ship then you get 12000 credits.
Just a idea, not the best one though
You could add other missions too, I am really exited for the mod
EDIT: With the mission, perhaps add a SP mission system where you go to a station and they tell you a mission descrioption and you can choose and stuff.
If not possible, just add it to the mission list (If you do decide to add it
)