Interesting idea.
I think it could be done.Have a website in which you just enter some post battle numbers and continue from there.
This MP campaign would have a map that changes based on battle data.
For example:
1. Rome attacks region x which is defended by Carthage.(you will pick a map that makes sense for that battle/region)
2. Rome has x amount of units and Carthage has x amount of units.(you will use those numbers to set up the battle)
3. Rome or Carthage wins. (remaining units will be counted for both sides and surviving units will remain in the game at full strength - because its more fun and simple that way)
This idea could support multiple players and all factions, it would still be turn based.
The economy would be simple, maybe 2 armies for every 5 regions.
Other rules would definitely work, as would specific scenarios.
You could definitely automate a lot of this.