First post! Yay!
When I say I want to play barbarians, I don't mean I want to play all the red units on the map. What I want is to have the challenge of starting on a map with developed computer civilizations, with a number of unsupported units equal to your typical barbarian horde. I then want to use that selection of units to quick-conquer a computer civ.
I doubt the devs thought of a play mode like this, though I would love it if you guys informed me that I was wrong.
But, if the game doesn't support a play mode like this, I have an idea: a lost game exchange. What I mean is this. If others are interested in this sort of play mode, those of us who actually play until we lose completely (because we suck

) can save the game *just before* we lose completely (like, the turn before). Then, open it with the editor, look for a full sized barbarian horde, preferably a bit away from the nearest developed civ. If there is one, turn all the reds to the civ that is about to get obliterated, convert the one city of that civ to the conquering civ, and save.
Of course, already knowing the map, you would get the wonderful fun that is exploring this brave new world. Thus, you *trade* this edited savegame with someone else who has done the same, and viola! You are now the newly elected leader of a bunch of backwater tribals destined to conquer an empire!