I think you're right about the Kingdoms mods, LB is probably the way to go. However I really don't see the mongols being much of a problem in Crusades. I don't think they came in till around turn 60 and they spent a least 10 turns just sitting around in the desert outside of Baghdad. The real problem to settle is the Venetians. They spawn early in the game and at Greeces backside, more then likely forcing the Greeks to maintain a large force far from the front line during an important time of expansion.

I think a way that might make the Venetian situation interesting (although I don't know how easy it would be to do) is to have them player controlled and spawn in boats in the middle of the Mediterranean so that anyone could be fair game for their invasion ;)