Yeah, I like the sound of this. And you're right, most games never do these kind of things (only one I can think that does is Guild Wars 2
It might be hard to put in, but maybe not if it's done slowly over time. The campaigns already have a few missions where the races overlap (like with the wedding missions); it wouldn't be too outrageous to have missions like these where you fighting a player from another race. Depending on who wins, their race gets stuff. Over time, you can add more of these missions (which should be independent of the campaign), and add more and more rewards for winning and punishments for losing (like the thing with the borders).
And having multiple players being called to a city at the same time, and then having to defend it from a bunch of other players, is a great idea for one of these missions.
Wonder what the devs will think of this...