Well personally, if I were a creative director at 343 and I got a mandate from Microsoft to make a new Halo trilogy continuing directly after the end of H3, I would try and come up with an entirely new enemy and threat in a brand-new kind of setting. I would try my hardest to keep away from the Covenant and the Flood, and look toward other elements of the fiction to find new styles of environments to explore outside of the standard Covenant purples, Halo greens, and Forerunner greys that so widely populated the original games. I would try and pick out the most basic elements that make Halo's combat unique - a carefully crafted set of conventional and exotic weaponry, each with a specifically designed function; the seamless integration of vehicular elements; a variety of enemy types, some intelligent, some not, each requiring different strategies to effectively combat; etc. - and find a way to take those elements and rework them into something new and original.

But that's just me.

E: oh and I'd also release it on PC the same day as the 360 version with a full-fledged editing kit and no GFWL, but again, that's just me