Possible ironman games/challenges:

Byzantines, Late period, no mercenaries.

Egyptians, High period, GA mode. This is fun if everyone decides to crusade at you, but the AI sometimes doesn't bother.

King of Jerusalem: French, high period, you must abandon France and expand from the crusader kingdoms.

Bridge lover mode: If any province has a bridge on one of its borders, you can only take it from that side.

No trading. (Ouch. At least you wouldn't have 20 governors getting corruption virtues every turn.)