I'm not sure if I quite explained the idea. It isn't that you are a corporation, like some capitalist-themed AW clone. Rather, the corporations are separate entities from the players, with their own armies and bank accounts. You, the player, buy shares in a corporation (or form a new one) to control its units on the map. (Of course, you can always buy just some shares to get a portion of the dividends, or hope that its price will raise...) But it's still divorced from you in such a way that someone else could buy a controlling stake out from under you, or it could just go bankrupt and your own personal filthy lucre is untouched.
I'm thinking of the 18xx series and the board game Imperial, which work this way.
In, re: attacking "your own" cities, I was imagining the game just didn't allow a corporation to attack the place that contracted it. But there'd be nothing stopping you from owning two corporations, one of which sends its entire army out to fight your personal enemies, while the other comes and invades the previous corporation's undefended city. I'm imagining there would have to be some form of insurance bond or penalty for failing a contract to prevent this from being too exploitable.