I definately like the suggestion of barbarian cities transforming into terrorist camps by, say, industrial era. By now what we have heared is that the best barb unit will be on line with the units of the least advanced civ. In my opinion it's wrong. Barbs with tanks and mechs? I think a terrorist unit of moderate strength is the best solution.
Also, though I agree with Banty that terrorists have a veeery little chance of being implemented. But there is nothing wrong in including terrorists in the game imo. Pirates / Prevateers were actually the old days "terrorists". And they are become almost "romantic" now

) Imagine in 200 years people will look back on Al-Qaida and make movies of them etc :S Also, there are lots of famous games featuring terrorists. Counter Strike..?