There was very similar thread some time ago.
This mode would become repetitive rather soon I think at least after "gold" combination of units would be found. Basically you would pick best units in their classes like raider -> bandit, riot -> oplaw(as it is), arty -> pillager and so on.
But dunno if there would be like 5 units and no more (including air) then it could become fun, but also it could turn into porcbattle