If the game could implement Battle for Middle Earth-style battalions, then the game could offer a "Battle Mode", where the player gets XYZ resources (determined by the host) to choose their army composition and then battle it out on a custom map against enemy players doing the same. No building. No resource gathering. Just tactical warfare. Essentially, it would be Rome:Total War style battles with 0 A.D. units. This would complement the game's standard mode, not replace it.
What would be needed to make this work I think is a bunch of UI work, UnitAI updates, a BattleAI player, plus battalions of course. The maps you battle out on can either by skirmish maps or random map scripts designed for the purpose. With random map scripts, the host can determine the size of the battlefield, so that's good, and maybe a few other variables (like season, weather, time of day, etc.).