I think they should focus on Muslim expansion in their next title, so that's spanning from 630, the founding of Islam to maybe 1400, when Tamerlane's invasion really brings about the downfall of Islamic superiority. Sure, we may all remember that North Africa and the Middle East were major areas of spread, but let's be honest, who else knows of the Ghurids, or the Ghaznavids before them, or the Samanids before them, or the Saffarids before them? Territories in India, the Middle East, North Africa, maybe Spain were changing hands constantly. Islamic areas were highly unstable and extremely rich during this time, which sounds like clear Total War to me.