I don't think we know quite how large battle maps can be yet (population wise). However, I would think that one way to incorporate clan vs clan battles, in the future, would be to enable a certain number of players on the same map at the same time (say 3 from each side) but with a 'draw limit' on the visual draw area within the map. Basically, you start a bit farther apart from each other and, if units are too far from you, you cannot see them on your screen. This would help with lag issues.
I don't think there is any way to get the entirety of a clan's population vs another clan's population on a single map.. the lag would be huge. However, it might be possible to allow for such large battles as above.
Also, they could institute some features to make 'clan wars' possible as long-term things. For instance, there could be an in-game scoreboard that automatically keeps track of the victories/losses between members of those clans. Also, the cost for 'withdrawing' from a battle against your enemy clan could be increased - thus giving incentive for one side to forfeit the battle once they have been defeated. There could be 'stakes' for the war - so that when one side wins they receive a certain amount of loot from the members of the losing clan.
"He uttered the word of curse, crush him with our Orcish iron!"