Some games I thought of:
Game 1 - Halo: Mythos
Campaign-only game with 21 missions telling different stories in the Halo universe
Missions 1-3: Exploring the history of the Precursors, Forerunners and humans.
Missions 4-6: The story of a guy who joins the Insurrection.
Missions 7-9: Takes place on Harvest during first contact with the Covenant.
Missions 10-12: The war from the perspective of a Sangheili general.
Missions 13-15: Explores post-war Sangheilios.
Missions 16-18: Adaptations of Homecoming, Mona Lisa, and Pariah.
Missions 19-21: A story about a group of ODSTs who are recruited into the SPARTAN-IV program.
Game 2 - Halo: Multiplayer Evolved
A multiplayer-only game that lets players create highly customized gametypes using Megalo and custom maps that incorporate custom vehicles, weapons, and more. Any user-made weapon, vehicle, Equipment, or AA can be inserted into any custom map, allowing an near infinite number of possible games. There will also be a global Spartan and Elite armour customization system, and several other species (Knights, Brutes, ODSTs, Marines, etc) you can play as, but with only limited customization. There will also be a system where players can monetize their gametypes, maps, weapons, vehicles, etc, and players who donate a certain amount will get an image of the map on a page in their Service Record or something like that. Anything from a giant space map reminiscent of Sandtrap but with warships instead of Elephants to a Mario Kart-style racing game would be possible. Also, the ability to have up to 64 players at once would be cool.
I don't have any more game ideas other than this.