Third Age does something similar for Medieval 2: Total War, and it does it very well (making it onto Tom's list of essential Total War mods a while back).
The grand strategy and large-scale battles of the Total War series make it ripe for Lord of the Rings conversions, and Rise of Mordor isn't the first to attempt it. First, the team will finish off the factions and add custom settlements into the mod. The background story of the campaign will be 'accurate to the books', but could still be a while off yet.