A really fun concept would be a co-op/team mode wherein two players control the same Civ: one controlling all eco based units and buildings and the other controlling all military based units and buildings. It would require communication from the teammates but could allow for some really interesting gameplay. Imagine ranked teams like this.
It could also be appealing to newer players as one individual can focus on and learn macro eco wise while another focuses on micro military wise.
I think the idea could be fleshed out a lot more, but the more I think about how it could look, it could be really fun.