You can already do this, in essence, in game. You just don't get the class name.
GLA 20, THM 20, CON 20.
Take your gladiator. Add some of the stat debuffs from THM (18), a nuke/shock spikes (CON), or Styrgian Spikes (THM), Bio (THM), Elemental DoTs (CON) and you have... a DRK.
Take different skills... and you get a PLD. The idea behind the armory system is that there is no need for advanced jobs because you can take the abilities from jobs you have leveled and create your own job, of sorts.
After rereading that it comes off a little more critical than I meant for it to be. To add something constructive, perhaps they could add "advanced classes" that are, essentially blank trait wise but have a few extra spell/skill ability/etc. points to spend and they could have their own unique traits to learn as well. How about that for a fairly easy to add compromise?
Edited, Jan 1st 2011 12:10pm by shinichoco