I would really like to see Vietnam handled at the political and or theater level. Vietnam 1965 was ok but I'd like to see a game put you in the shoes of LBJ,Nixon, Westmoreland, or Macnamara. I can't think of any PC games that have done so in recent memory.
I'd also love to see a PC adaptation for the Iran-Iraq war. That war is fascinating at so many levels and is a rare example of a "modern" war with "modern" technology being fought at a near total war level over an extended period of time. Most modern wars are either COIN or a big country beating up on a little one. Iran-Iraq is modern technology and roughly equal opponents with some really interesting political dynamics. The problem with that game is there's no "good guy" so no one wants to touch it because there's no one anyone wants to play as.