I would love to see Metal Storms(google it, it's wicked), stealth tanks(like PL-01), orbital bombardments(Missile silo in space), satellites gameplay(like Civ Beyond Earth), power armor... anything that is grounded in science and theoretically possible. There are a lot of articles and videos out there on the matter.
PS. I would live to see Digital Age, Information Age etc.
PS2. You can also go back in time introducing new gameplay mechanics in the form of an age update(Age of Colonization for example giving all of us a more interactive single player experience)