I like this idea as it's similar to what I think will happen (easy to add with the current system and not wasting people current progress).
Instead of a spec tree or whatever grid system for the new unlocked classes, why not just use the existing Guild Mark system ? You gather Guild Mark in a similar fashion (new guildleves at the many unused r50 camps, quests, instances, etc ...) and use them to buy active/passive actions/traits for the new advanced class (keeping your AF gear and other ideas).
That would make something to do for the people already r50 and isn't your typical grinding.
