could be cool but it would offcourse need to be a yes or no event.. because some people don't care about lore and might want to keep an alliance with rohan..
i actually think a very difficult script for isengard with a choice to become evil instead of being evil from the start is way cooler with 2 unit rosters.. one with good merc units from different factions.. and the other the normal uruk/wildmen unit roster..