Since the current Halo trilogy goes deeper into the story of the Forunners & all that stuff 100,000s of years ago, how cool would it be if when 343 is done with the Master Chief saga they make a Halo game that occurs during the Forerunner-Precursor Wars or Forerunner-Human wars. There’s so much I want to learn about that time period and it would so cool to play as a 1st generation human or Forerunner.
You would be the most OP -Yoink!-er ever. I like it 
I honestly think it would be better as an RTS. With a war of THAT scope going on a first person shooter just doesn’t do it justice. Maybe a SINS style game… with ground combat… would be cool.
It could just be period during the war, ie a battle or other task. The war doesn’t have to be solved in the games, especially since we already know how they turn out. It’s a time period that would be very fascinating to go into to. We could see Precursors 
and maybe get a TRUE answer to how the Flood came to be
It would be interesting to see the humans get their butts blasted back to the stone age.
I would be excited to see this as a standalone firefight game or RTS.