Here are some ideas, How about enemies of the forerunnurs who took over a Dyson Sphere (who killed the inhabitants) awaking from suspended animation, used forerunner technology to create starships, and used flash clones of humans and covenant species to reclaim the galaxy? The flash clones could be used as tactical insertions into each ruling domain to cause maximum damage and confusion and to get the Covenant and Humans to fight again then attack. Or Seeing how large our galaxy is.... How about an alien empire that dwarfs the Covenant Empire and the UNSC? The UNSC cannot field that much personnel so they use forerunner technology to improve on genetically enhanced flash clones (creating clones that also live longer) and imprint each clone with information (according to need) such as: Mechanic, Starship Engineers, Soldiers, Medical Doctors, Scientists ect... The following breakthrough would have secondary consequences, people of the colonies would revolt as thoughts of humanity would be replaced with flash clones with instant knowledge of careers that would take years and years for a regular person to master. The relationship with the Sangheili would become complicated but would have no choice to fight with us to prevent them from being conquered by the new threat. The other species of the Covenant would aid the new threat and be absorbed by them making the new threat much harder to beat. I would also include more ship to ship space battles, using point defense weapons, warfare linkage between ships to coordinate PDW's and MAC/Plasma/Missile barrages, along with using fighter craft to attack in the enemy fleet. I would also include heavy ground warefare using believable ground tactics. I dunno... Think "Band of Brothers" and "Midway (1977)" but in outer space.