I really like your idea, but there is a few things about it I would change.

First off lets give the current, or base, classes a level cap of say 30. Then at level 30 you can take quests to unlock advanced classes. Once the quest is complete you get a weapon for the advanced class and switching to it will be done by switching weapons. Now you become the advanced class and you start at level 30 as that class and continue leveling until the cap.

Now if you switch to a new advanced class you start out at level 30 again. Like your idea all advanced class skills can only be used by that class, but all basic class skills can be used by anyone. Talent Tree, License Board or whatever can stay, but you will not be able to put points into the other class. Because of that the talent tree should be broader and have more on it, making the class even more unique.

Really my idea isn't much different than yours, I just think advanced classes should be treated as their own separate class instead of a spec within a class.

Also sorry if this was posted already 20+ pages is a lot to read.