People suggested completing a short campaign (15 provinces for victory?) if you are in a hurry to play as the barbarians as the full Imperial one (50 provinces) is rather an epic.

Also, if you wipe out a given action as Romans, I believe you unlock it.

Easiest thing might be modding as was suggested - it's a question of a single text file, IIRC. I can't recall the specifics, but a search "unlocking factions" or some such should dig up a thread that gives the required info. (Or a more informed person might chip in here).

I would say that the Roman campaigns are some of the most fun, however, if only because of the Senate missions and greater variety of General's speeches. Playing barbarians can seem a little quiet by contrast.