Russia and China(Han) will be the main enemies in at least one age. Since the Civ3 Chinese Rider is essentially the Mongolian UU, it might be an option to just take him from Han and give him to us. Whatever the UU will be, it should be fast, cheap, and available early without the need of iron.
Expansion could be forced by the complete absence of both fresh water and grasslands in Mongolia. This would require 2-3 super-resources in our capital radius (e.g. Mongolian Horses, on plains: +3f/+2s/+1c), to enable a bearable settler and military production rate...could be an interesting challenge indeed, although difficulty then might better be set to Monarch or below. Also different corruption rules would be cool - either lower it significantly for one or all government types, or introduce a medieval Communism. (Better yet: make corruption rate proportional to the city's population - but I doubt that this is possible via txt editing
).
The other civs (apart from Russia & China) depend on the scope of the map - I think nearly everything is possible here.
Option (a): Small to medium, pangea, no ocean at all, map borders are Ural, Himalaya, extensive tundras and in the east the deltas of Yangtse et al.. No other "great" civs, early wars take place between Mongolian tribes. VC: Conquest.
Option (b): Same as (a) but with the addition of quite large southern territories plus the Indian Ocean, so that both India and Babylon/Persia play a major role during most of the game (and not only as luxury provider). Korea might also be granted a smaller part in the NE. Map size: Medium to large. VC: Domination.
Option (c): The next enhancement would be either Japan or (and?) better Europe. Europe would be the perfect candidate for the highly advanced but not too powerful bunch of nations, provided the Ural and perhaps a (more or less fictitiuos) wide desert can seperate them from Asia long enough...(I wonder if it will be possible thus to create a situation where the hordes of medieval riders prevail against rifleman and conquer Europe
...) Egypt might play a smaller role SW in this setup. Map size: Large (perhaps the tech costs need to be raised). VC: Domination.
Option (d): Guess what? Yep, the world - or a least the complete northern hemisphere. Map size had to be huge, and even Domination might be too tedious. Probably not the best setup, since both tech speed and corruption would soon enter ridiculous dimensions. (Nonetheless this could be one of the more interesting World Games...)
ability. You just have to walk with them right into an enemy city, and the population, defenders, territory and your rep will be dead next turn. 
