Honestly Halo should just be done by this point. We don't need "forever franchises" where people are playing the same damn game for ten years. Halo had its hay day with the first 5 games made by Bungie, and they were honestly perfect.
I don’t agree, I think there is plenty of room for interesting stories in the universe. You could do a prequel set during the rainforest wars, or a game centered around first contact with the Covenant at Harvest. My personal favorite pet idea is a a Dead Space like game where you’re stuck in a flood infected ship a la the short story Mona Lisa.