This suggestion would be a really, really, out of the box suggestion, (and probably really unlikely) but what if that 9th civ was one that each player created? What i mean is you could pick a name, make a city list, choose a leader name, choose colors, as long as another civ didn't have these colors, you could pick from a set of generic leaderheads or flags, and then you could make your own set of UA, UU, and UB/UI. You could pick from existing ones, or pick from a set of components to make your own. You could change your civ at any time, and you could play as it!
It would fit the achievement list theory, because Your Civ would be in between P-Z.
Even though it is almost certain to not be in, i think it still could be really fun.