In a game of this nature I'm not sure how many people would pay for cosmetic items you could lose at any moment. I think they could pull it off if the cosmetic microtransactions were simply skins for existing items and they were account bound, so you could never lose them; for example, if you bought a blue cape skin for your ranger capes, then whenever you equipped a ranger cape on any of your characters, it would look blue instead of green.
If I'm not mistaken they once talked about making it so you could only play for X hours a day / X days a month if you were a free player. I think that would be a decent model also, since it wouldn't force them to add flamboyant cosmetic items to the game to make them money (which is what inevitably happens in every game with cosmetic microtransactions.) Plus it would disencourage alts.