It could also be a DLC for the Papacy.
Great suggestion, but as the base title suggests, the game is supposed to be based around the Crusaders (Catholics) and this system is fairly well developed. If they did improve upon this, I would hope it would be in the form of a patch, not a DLC.
could be an HRE DLC for all we know.
Another interesting thought. I would welcome such with open arms but I doubt that's what they would label as the Legacy of Rome. The HRE was only Roman in name.
A Byzantine expansion does not interest me at all. An overhaul of the Orthodox religion would be great, but in a patch. That should have been included in the base game. To sell the full version of the Orthodox religion in a DLC would be the cheapest thing I've ever seen. After all, all Christians took part in the crusades.