What about the opposite: diversifying the classes. Keep the amount of types reasonable, but allow for customization. A good idea would be to
treat classes as stats that can be modified by certain items. Say you have a sword. It could add to a warrior stat where a staff may add mage points. Other choices might include color-coded rings or even armlets or other relatively unisex jewelry. The following petition is campaigning for crystals in different colors that might equip as a crown, belt, bracelet, etc. that could prove very useful to that end.
http://www.gaiaonline.com/forum/viewtopic.php?t=10234784 (Yes, I'm aware it suggests elements, but I hold respect for the idea and even suggested a name for them when I first found it.) This system would allow for easy personalization with a minimum of major changes to an avatar's appearance while meeting several requests for accessories. Also, it would allow maximum basic user access to classes for cheaper items and
leave room for advancement via upgrades or more expensive pieces of equipment. In short, it offers availability, customization, and flexibility.