Hmm, i personally think this would be a great idea. maybe not a serious game, maybe like a DLC to reach (( or not lol )).
But i dont think that the player should play as johnson or bryne, possibly jenkins or some other soldier. and in the end depending on how well you did during the campaign, you would eather be left on the planet while it gets glassed or you would watch from orbit as you escape, and if they really wanted to make it longer, have you come back to harvest when cole came back and fight on the ground taking it back